The present invention relates to aspartame-containing acidic beverages, and also relates to acidic beverages designed to prevent a storage-induced reduction in the sweetness of aspartame, which comprise a polymerized polyphenol (particularly a polymerized catechin). The present invention further relates to a method for preventing a storage-induced reduction in the sweetness of aspartame in aspartame-containing acidic beverages by using a polymerized polyphenol (particularly a polymerized catechin).