Diet Pepsi is tweaking its formula to stay sweet a little longer.

PepsiCo Inc. is testing new artificial sweeteners that let the soda keep its taste for a longer period of time. The problem is that the current sweetener used in the soda _ aspartame _ loses its potency faster than high fructose corn syrup, the sweetener that's used in most regular sodas.

A person with knowledge of the situation says the company had considered importing versions of Diet Pepsi sold in other countries to the U.S. But now it's testing other sweetener mixes, with a new version set to come out as soon as next year.

The new version will use the same formula that creates Diet Pepsi's overall taste, according to the person, who requested anonymity because she wasn't authorized to speak publicly. But it will use a mix of artificial sweeteners, including acesulfame-potassium, or ace-K, that has a longer shelf life.

Aspartame on its own is more sensitive to heat, which is a problem when sodas are sitting in trucks or waiting to be shipped to retailers.

In an e-mailed statement, PepsiCo said that it's "always looking at ways to provide the best consumer experience," but that it has no plans to change the taste formula of Diet Pepsi.

It's not the first time PepsiCo is tweaking a diet soft drink. The company, based in Purchase, N.Y., made a similar switch to its Diet Mountain Dew in 2006. The Coca-Cola Co. tweaked the sweetener used Diet Sprite in 2000.

By blending artificial sweeteners, companies create a "synergistic effect" that prolongs the sweetener's potency, said John Sicher, publisher of the industry tracker Beverage Digest.

PepsiCo's latest tinkering comes as an increasing number of diet drinks have come on the market. Coca-Cola in 2005 introduced its Coke Zero, which is intended to taste more like the original Coke. Pepsi followed up two years alter with Pepsi Max. But Pepsi Max hasn't performed as strongly as Coke Zero.

After losing market share to Coke in recent years, PepsiCo is looking to bolster its flagship soda brand this year. The company is significantly boosting advertising for Pepsi, which two years ago was edged out by Diet Coke as the No. 2 soda in the country. Coke remains No. 1.

Diet Pepsi ranks as No. 7, according to Beverage Digest, and its sales volume last year was about half that of Diet Coke,.

A spokesman for Coca-Cola, Scott Williamson, said there are no changes planned for Diet Coke, which still uses only aspartame as a sweetener.

The U.S. Food and Drug Administration approved ace-K for use in 1998. It is often used in combination with other artificial sweeteners, according to the Center for Science in the Public Interest, a food safety advocacy group. The sweetener is used in a wide range of foods, including baked goods, chewing gum and gelatin desserts, as well as diet soft drinks.