"Those guys on Madison Avenue are so smart," she said, adding: "It's not as if they're saying Pepsi is good for you. It's a little more subtle."

Certainly, people don't like fizz-less soft drinks. And while aspartame, the artificial sweetener used in diet soft drinks that is marketed under the brand name Nutrasweet, can lose its sweetness over time, an estimated 97 to 98 percent of all such sodas are consumed before that occurs, primarily because soft drinks sell so fast they rarely lose their flavor.

"There's one extreme case I heard of a 7-Eleven in the Mojave Desert," said Michael Jacobson, co-founder of the Center for the Study of Commercialism, another Washington-based activist organization, "so it might be useful to consumers to have freshness dating on diet drinks."

The New York State Consumer Protection Board, in a statement yesterday, praised Pepsi for responding to its requests that go back to July 1992 for freshness dating on diet sodas.

Still, Mr. Jacobson expressed concern over whether "fresh might imply a healthfulness analogous to dairy products or fresh vegetables."

The Pepsi executives' efforts to make freshness a quality consumers will consider when choosing among soft drinks are an attempt to stir up what has been a flat market. Growth in the $48 billion American soft drink industry has slowed during the 1990's. Diet products, particularly, have been hurt by the growing popularity of "New Age" offerings like iced teas and flavored waters.

Not only has Diet Pepsi's performance lagged behind that of other diet brands, but Pepsi's most recent gimmick to effervesce sales -- the clear colas Crystal Pepsi and Diet Crystal Pepsi -- has been deemed a dud and is undergoing an overhaul.