Cellobiose phosphorylase from Cellvibrio gilvus was used to prepare 1,5-anhydro-4-O-beta-D-glucopyranosyl-D-fructose [beta Glc(1 -> 4)AF] from 1,5-anhydro-D-fructose and alpha-D-glucose 1-phosphate. beta Glc(1 -> 4)AF decomposed into D-glucose and ascopyrone T via beta-elimination. Higher pH and temperature caused faster decomposition. However, decomposition proceeded significantly even under mild conditions. For instance, the half-life of beta Glc(1 -> 4)AF was 17 h at 30 degrees C and pH 7.0. Because beta Glc(1 -> 4)AF is a mimic of cellulose, in which the C2 hydroxyl group is oxidized, such decomposition may occur in oxidized cellulose in nature. Here we propose a possible oxidizing pathway by which this occurs.