December 17/New York/Associated Press -- Diet Pepsi is quietly changing its sweetener ahead of a major rebranding of the soft drink set for next month.

The change comes as PepsiCo looks to reinvigorate its namesake brands after losing market share to Coca-Cola in recent years.

Cans of Diet Pepsi around the country now list a mix of two artificial sweeteners, a pairing commonly found in newer diet sodas. Previously, Diet Pepsi used only aspartame, which is sensitive to heat and breaks down more easily.

This summer, PepsiCo declined to say whether it would go ahead with such a change after reports surfaced that it was testing the new sweeteners. Although the switch is only intended to help prevent the taste from degrading over time, companies are often sensitive to public perceptions that they might be tinkering with major brands. PepsiCo executives likely don't want to call any attention to the use of artificial sweeteners in the drink either.

When reached for comment, PepsiCo spokeswoman Andrea Canabal said Diet Pepsi using the new sweetener mix started hitting shelves in early December. She said the new mix will be more widely available in the coming weeks.

"It's not like a light switch. It'll start appearing as shelf space clears," she said. In January, Canabal said the company is planning a major ad campaign that will include a new logo with a heart and the theme "Love Every Sip."

The sweetener change will not be explicitly communicated in the ads, which will feature actress Sofia Vergara.

In addition to aspartame, cans of Diet Pepsi found in New York, Omaha, Neb., and the San Francisco Bay Area now list acesulfame potassium as an ingredient. The ingredient is often used in combination with other artificial sweeteners and can be found in a wide range of foods including baked goods, chewing gum and gelatin desserts.

PepsiCo said in a statement Sunday that it is adding a "very small amount" of acesulfame potassium "to ensure consistency with every sip." The sweeteners used in Diet Pepsi vary depending on the region of the world.

The move to improve Diet Pepsi comes amid a broader push by PepsiCo to boost sales of its flagship soda. Under pressure from investors, CEO Indra Nooyi earlier this year announced the company would step up investment in its flagship brands.

Diet Pepsi rakes in roughly $5 billion in a year in revenue and remains one of PepsiCo's biggest moneymakers. The company, based in Purchase, N.Y., also makes Frito-Lay snacks, Tropicana juices and Quaker oatmeal.