Aspergillus niger glucoamylase undergoes complex thermal inactivation phenomena, as revealed by the two-slope pattern of the enzyme. logarithmic activity versus time curves. Two proposed general inactivation models are shown to tit enzyme inactivation data reasonably well. They are as follows: parallel isozyme model which assumes the presence of two different active forms of glucoamylase and two models of inactivation by a series mechanism, involves intermediate, temperature dependent and less active forms of the enzyme. Discrimination between these models was based an dependence of kinetic constants on temperature as well as the analysis of possible structure of the enzyme intermediate forms.