The activity of pectin methyl esterase (PE) in potatoes and carrots during blanching treatments was modelled and analysed. Two iso-enzymes were found to exist, one bound and one soluble. The bound form can be converted into the soluble form. These iso-enzymes have a different susceptibility to conversion and heat denaturation. All rate constants of reaction were modelled as dependent on temperature according to Arrhenius＇ law. Regardless of the daily variance in measuring the series the variance accounted for by the model in multivariate nonlinear regression analysis was more than 90% for both the individual activity data and the mean data. The same model formulation could be applied for activities assayed against pectin of 8% and of 68% esterification. The parameter values obtained were highly comparable in both analyses.