PepsiCo Inc. spent two years surveying consumers, conducting taste tests, and tinkering with soft drink recipes in order to fix Diet Pepsi, only to find it had completely misread what its customers wanted.

Less than a year after launching its new Diet Pepsi with sucralose—and abandoning the controversial sweetener aspartame—PepsiCo has made an about-face. On Monday, after plummeting sales and howls of protest from the diet soda’s devotees, the company announced it will bring back Diet Pepsi with aspartame in September.

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