Acid-catalyzed hydrolysis of cellulose is a starting point for producing glucose-based fuels and chemicals. According to the mechanism of cellulose hydrolysis, hydrogen ions are responsible for the reaction. This study aims to elucidate the phenomena in cellulose hydrolysis at 180-220 degrees C using 5-20% (w/w) formic acid or 0.09-0.50% (w/w) sulfuric acid as catalyst. A significant difference between the reaction rates in formic and sulfuric acids was found despite the same hydrogen ion concentration. The findings could not be explained unambiguously through changes in the crystallinity of cellulose. It is suggested that, in addition to hydrogen ions, cellulose hydrolysis is affected by other species found in reaction media. A kinetic model based on specific acid catalysis combined with the effect of bisulfate ions was developed for cellulose hydrolysis in sulfuric acid.