A sugar substitute named aspartame (commonly known by the brand name NutraSweet) is usually used instead of sugar to give diet soda a sweet taste. Opinion is mixed as to the taste of these beverages: some think they lack the taste of their sugar-sweetened counterparts, others think the taste is similar. Some also note an unusual non sugary aftertaste.

Some feel the opposite, that diet pop has no aftertaste and that high fructose corn syrup sweetened soda has a gritty over sweet aftertaste.

Cyclamates

The first artificial sweeteners used in diet soda were cyclamates (often synergistically with saccharin). While many say these cyclamate-sweetened soda had a more pleasant taste than the diet soda that followed them, in 1970 the Food and Drug Administration banned cyclamates in the United States on evidence that they caused cancer in lab rats. Cyclamates are still used in many countries around the world, including for diet soda. (See cyclamate.)

Saccharin

Once cyclamates were banned, American producers turned to saccharin. Saccharin alone was often criticized for having a bitter taste and "chemical" aftertaste. Some manufacturers, such as Coca-Cola with Tab, attempted to rectify this by adding a small amount of sugar. In 1977, the FDA was petitioned to ban saccharin, too, as a carcinogen, but a moratorium was placed on the ban until studies were conducted. The ban was lifted in 1991, but by that time, virtually all diet soda production had shifted to using aspartame. Perhaps the most notable holdout is Tab, which nevertheless also uses some aspartame in its formula.

Aspartame

Aspartame had the great commercial advantage of not being linked in the public mind with cancer, and by the mid-1980s, American manufacturers were switching en masse from saccharin to aspartame. The 1982 introduction of aspartame-sweetened Diet Coke accelerated this trend. Today, at least in the United States, "diet" is nearly synonymous with the use of aspartame in beverages.

Sucralose and acesulfame potassium; "sugar-free" sodas

Recently two other sweeteners, sucralose (marketed as Splenda) and Acesulfame potassium ("Sunett" or "Ace K", which is usually used in conjunction with aspartame, sucralose, or saccharin rather than alone) have come into growing use, particularly by smaller beverage producers (e.g. Big Red). Diet Rite is the non-aspartame diet soda brand with the highest sales today; it uses a combination of sucralose and acesulfame potassium.

Advocates say drinks employing these sweeteners have a more natural sugar-like taste than those made just with aspartame and do not have a strong aftertaste. The newer aspartame-free drinks can also be safely consumed by phenylketonurics, because they do not contain phenylalanine. Critics say the taste is not better, merely different, or note that the long-term health risks of all or certain artificial sweeteners is unclear.

The widespread, though not universal, agreement that the newest formulations taste much more "normal" and sugar-like than the older diet sodas have prompted some producers, such as Jones Soda, to abandon the "diet" label entirely in favor of "sugar-free soda," implying that the taste is good enough to drink the soda even when not trying to lose weight. (This idea was first floated by Diet Coke in 1984, with the tagline, "Just For the Taste of It.")

In 2005, the Coca-Cola Company announced it would produce a new formulation of Diet Coke sweetened with sucralose, to be called Diet Coke with Splenda, but it would continue to produce the aspartame version as well. There were also rumors that a sugar-free version of Coca-Cola Classic, also sweetened with sucralose, was being formulated as well. This formulation was eventually called Coca-Cola Zero.