Michael J. Fox. Michael Jackson. Madonna (for one brief flickering moment). While second-largest soft-drink concern PepsiCo (NYSE: PEP) isn't recruiting its 80s spokespersons back to the fray, the company is bringing back the old-fashioned taste test, in an effort to prove the flavor of Diet Pepsi superior to that of the better-selling Diet Coke - the second-most popular brand under the Coca-Cola (NYSE: KO) umbrella.

Starting today, PepsiCo will be handing out samples of Diet Pepsi, in newly designed cans, and boasting the claim that their calorie-free product has "more cola taste." A recent taste test, reminiscent of the familiar "Pepsi Challenge" from days of yore, yielded positive results, with 56% of respondents believing Diet Pepsi tasted more like "real" (full-calorie) cola, while 41% preferred Diet Coke.

Sales of diet soda are on the decline, as sports drinks and bottled water gain popularity, but the Diet Pepsi brand is losing momentum at a faster clip than Diet Coke. According to an article in today's Wall Street Journal, unit case volume of Diet Coke slipped 0.1% in 2006; Diet Pepsi volume dropped 1.0% last year.

The once-proclaimed "choice of a new generation" has an uphill battle ahead of it . In our "Battle of the Brands" feature last month, Pepsi was felled by Coke, 41% to 59%.

Maybe the Diet brand will have better luck.

Beth Gaston Moon is an analyst at Schaeffer's Investment Research.