We measured the relationship between measured activity concentration and temperature for alpha-amylase (EC 220.127.116.11), using 10 different substrates. At 25-37 degrees C the Arrhenius plot was linear. The activation energy ranged from 33.3 kJ.mol-1 with maltoheptaose as substrate to 54.4 kJ.mol-1 with the Blue-Starch method. Activation energy was lowest for substrates having seven glucose moieties, the ones most suitable for determining alpha-amylase--i.e., the presence of more or fewer glycosyl units increased the activation energy. Substrates with blocked groups at the nonreducing end of the oligosaccharide chain were not considered here, because the relative reaction rates obtained with these substrates were less than those obtained with nonblocked substrates. We also determined temperature-conversion factors for alpha-amylase reaction with the various substrates. The results are discussed in relation to thermodynamic parameters of some other enzymes, e.g., creatine kinase and alkaline phosphatase.