To save the planet, we've been told to stop burning coal, ditch our gas guzzlers and switch our light bulbs to energy-efficient CFLs. Here's something else to worry about -- the vending machine down the hall.

Yes, the vending machine. Vending machines and commercial coolers that keep drinks cold run around-the-clock, rely on inefficient compressors and, worst of all, use HFCs, a potent greenhouse gas. So when Greenpeace challenged The Coca-Cola Co. to get rid of conventional vending machines and coolers, the world's largest beverage company promised to develop vending machines and coolers that are 40 to 50 percent more efficient and HFC-free.

Since then, Coke has invested $40 million in research and testing, published a 900-page technical study and organized a coalition of companies that sell cold drinks and ice cream, including Unilever, McDonald's and (gasp!) PepsiCo. to attack the problem. Last year, at the World Economic Forum in Davos, Coke declared victory: E. Neville Isdell, the company's chairman and CEO, and Gerd Leipold, who leads Greenpeace, unveiled a new, HFC-free, super-efficient vending machine. About 8,000 of the climate-friendly machines have been deployed, most to high-profile venues like Davos and the 2008 Summer Olympics in Beijing.

That's good for Coke because its reputation is so important. "The name on this building is the name on the bottle," Isdell says. But all of its hard work hasn't done much for the planet, at least not yet: Coca-Cola and its bottlers still have about 10 million old-style coolers and vending machines deployed in the 210 countries where its products are sold. That 's because its bottlers, who are independently owned, don't want to pay extra for HFC-free units. And manufacturers,who have lots of capital sunk into their current production lines, have yet to produce them in bulk. "We have the technology and we know it works," Isdell says. "The problem is, the economic logic doesn't hang together."

This is not just a problem for Coke. Many big companies -- GE, Wal-Mart, GM, IBM, Citi and Procter & Gamble--say they want to help save the planet, and they mean it. But doing so is not as easy as changing a light bulb. Either their customers won't go along, or their distributors balk, or a clean technology they want to deploy is not ready for prime time. This story is about the gap between good intentions and meaningful results.

Examples abound: GE advertises coal-burning power plants that sequester carbon dioxide but it has yet to sell a single one. P&G says its customers don't want recycled stock in Bounty paper towels or Charmin toilet tissue because the quality suffers. Despite its sophisticated efforts to save energy, Wal-Mart's greenhouse gasemissions are growing.

Why look at Coke? In part, because its commitment starts at the top. The 64-year-old Isdell fell in love with the outdoors as a child growing up in Africa. "I'm an early convert to the environmental movement," he told FORTUNE, in an interview about all things green. He has had a keen sense of social justice since protesting apartheid as a student in Cape Town. More recently, the soft-spoken executive developed a passion for wildlife photography and gave up eating meat.

Inside Coke, a team of a half dozen or so environmental advocates, led by a former Clinton administration official named Jeff Seabright, operate almost as an in-house NGO. They get high marks from outsiders who work with the company. "The inspiration and the perspiration are real," says Kert Davies, research director at Greenpeace USA.

Coke has good business reasons to take sustainability seriously. Its brand is all-important. It depends on clean water, a scarce resource. And the company has been stung by alleged misdeeds. Although no one ever proved a connection, a Coca-Cola bottling plant in Kerala, India, was shut down in 2004 after nearby wells went dry. That fed into a global anti-Coke campaign that caused about 20 colleges, including the University of Michigan and NYU, to suspend the sale of Coca-Cola products, albeit temporarily.

For all those reasons, Isdell has made the environment a touchstone of his four-year tenure as CEO. Coke's sales, profits and stock price have all grown since he came out of retirement to take over; he will turn the chief executive's job over to Muhtar Kent in July, and remain chairman for another year. Under Isdell, Coke has set bold environmental goals -- to become "water neutral," to come up with a sustainable package for its beverages and to help curb climate change. Let's see how he's doing.

Trickle-down Economics

Last winter, Isdell's desire to turn Coke green took him to the West African country of Mali for the first time. "That was country No.134," he says. "They are one of the African countries that is improving pretty rapidly -- good GDP growth, good governance." In the capital of Bamako, Africa's fastest-growing city, some people blamed Coke for polluting a stream that ran by its bottling plant.

Like most of the 900 plants in the Coca-Cola system, the Bamako bottling plant is independently owned. Its discharge met government standards, the stream was polluted anyway and the owner said he didn't want to spend the money for a new water treatment system.

"So, then, as a business, you're faced with two choices," Isdellsays. "You can go on making that very defensive argument, or you can take a totally new approach."

Coke provided the bottler with money -- the company won't say how much -- to finance new equipment. Waste water from the plant can now be used for irrigation. Beyond that, the company made a $280,000 grant that was matched by U.S. AID to improve the municipal water system. "We are now giving at least 22,000 people...access to safe water, clean water in Bamako," Isdell says.

Becoming "water neutral," as defined by Coke, has several elements. First, it will use water more efficiently. Second, it has pledged that by 2010 all of the water discharged from its bottling plants will be clean enough to support agriculture or aquatic life. (Right now, about 85 percent meet that standard.) Finally, the company will replenish the world's supply of fresh water by an amount equal to all the water included in its drinks; it will do so by supporting healthy watersheds (the catchment areas that feed streams and rivers) and underwriting clean-water projects around the world.

No company is doing more than Coke to provide clean water to the world's poor (and not-so-poor) people. With the Gates Foundation and the global nonprofit CARE, Coke is delivering water-purification systems to dozens of schools in Kenya. With local partners, it is building more than 300 rainwater-harvesting structures to capture the monsoon rain in India which otherwise goes to waste. Last year, the company pledged $20 million to a partnership with World Wildlife Fund to improve the health of seven river basins, including the Yangtze, the Mekong, the Danube and a network of streams, lakes and rivers in the southeastern U.S., Coke's backyard. Last fall, the drought in Georgia was so bad that the governor prayed for rain on the State Capitol steps.

Farther from home, Coke is leading efforts to help poor villagers save water in the highlands of Guatemala. There, Coke and WWF, with local businesses and nonprofits, are paying villagers who agree to use farming practices that conserve water, or to switch to smaller and more efficient wood stoves so that fewer trees are cut down. What does that have to do with Coke? It's literally trickle-down economics, intended to increase the flow of water to its bottling plant downstream. Not incidentally, increasing the flow of fresh water helps preserve the MesoAmerican reef, a vast but vulnerable hotbed of aquatic biodiversityoff the coast.