A thermokinetic reduced extent method for the product inhibition of single-substrate, enzyme-catalyzed reactions is proposed in this paper. By analyzing the calorimetric curves of these reactions, this method can be conveniently used to calculate both kinetic parameters (K-m, K-i and V-m) and molar reaction enthalpy (Delta(r)H(m)), and to establish the type of product inhibition simultaneously without adding product. The arginase-catalyzed hydrolysis of L-arginine has been studied by microcalorimetry and the product, L-ornithine, has been established as a competitive reversible inhibitor. The kinetic parameters calculated with this method are in agreement with those given in the literature.