A simple method is proposed to analyze the kinetics of thermal inactivation of enzyme systems which consist of two groups differing in their thermal stability. Using this approach kinetic parameters for thermal destruction for each fraction (the heat labile and heat resistant fraction) can be estimated directly from thermal treatment data. However, to determine the relative concentration of the fractions within the enzyme system, additional information concerning the enzyme assay used to determine enzyme activity is required. Commercially available horseradish peroxidase was used to test the applicability of the method. Analysis of data generated from thermal treatments of the peroxidase using the proposed method were found to be satisfactory. E, values of 21 kcal/mole and 34 kcal/mole were obtained for the heat resistant and heat labile horseradish peroxidase isozymes, respectively. D, 2 c values were 42 min and 1.2 min, respectively.