beta-Glucosidases (BGs) from Aspergillus fumigates, Aspergillus niger, Aspergillus oryzae, Chaetomium globosum, Emericella nidulans, Magnaporthe grisea, Neurospora crassa, and Penicillium brasilianum were purified to homogeneity, and analyzed by isothermal titration calorimetry with respect to their hydrolytic activity and its sensitivity to glucose (product) using cellobiose as substrate. Global non-linear regression of several reactions, with or without added glucose, to a product inhibition equation enabled the concurrent derivation of the kinetic parameters k(cat), K-m, and the apparent product inhibition constant K-app(i) for each of the enzymes. A more simple fit is not advisable to use as the determined K-app(i) are in the same range as their K-m for some of the tested BGs and produced glucose would in these cases interfere. The highest value for k(cat) was determined for A. fumigatus (768 s(-1)) and the lowest was a factor 9 less. K-m varied by a factor of 3 with the lowest value determined for C. globosum (0.95 mM). The measured K-app(i) varied a factor of 15; the hydrolytic activity of N. crassa being the most resistant to glucose with an apparent product inhibition constant of 10.1 mM. Determination of K-app(i) using cellobiose as substrate is important as it reflects to what extent the different BGs are hydrolytically active under industrial conditions where natural substrates are hydrolyzed and the final glucose concentrations are high. Biotechnol. Bioeng. 2010;107: 943-952. (C) 2010 Wiley Periodicals, Inc.