The alpha-amylase of Bacillus coagulans is a saccharifying alpha-amylase which hydrolyses the disaccharide maltose [L. Keating, C. Kelly, and W. Fogarty, Biochem. Soc. Trans., 24 (1996) 44S]. The pH maximum for maltose hydrolysis is pH 5.0, differing from the pH maximum for starch hydrolysis which is pH 6.0. Studies using reducing end 14C-labeled maltooligosaccharides revealed a substrate-dependent pH maximum shift; hydrolysis of radiolabeled maltotriose (G3*) was maximal at pH 5.0 while the pH maximum for hydrolysis of radiolabeled maltopentaose (G5*) and maltohexaose (G6*) was pH 6.0. With maltotetraose (G4*) however, the pH maximum was pH 5.0-6.0. In addition, the bond cleavage pattern of G4* was dependent on pH. At pH 5.0, the pH maximum for maltose hydrolysis, the frequency of hydrolysis of the reducing end terminal bond of G4* was maximal. Determination of the pH maximum of the productive binding modes of the cleavage patterns of G3* to G6* illustrated the possible role of the occupation of subsite r + 2 in the pH control mechanism of B. coagulans alpha-amylase.