The activity of endo-polygacturonase (PG) in peaches during storage at different constant temperatures is shown to be the result of a formation from some inactive predecessor and a denaturation or decay into an inactive form. This whole process strongly resembles the normally encountered turnover. On these premises, a process-oriented mathematical model is formulated. The variance accounted for by the model in multivariate nonlinear regression analysis is more than 80% for the data gathered in two successive seasons. Analysis of the data from both seasons combined did not decrease the descriptive power of the model, regardless the fact that the maturity at harvest and the initial level of enzyme activity showed major differences. The values of the parameters obtained from the data of both seasons combined were comparable with those of the individual seasons. The effect of the action of PG on the firmness of peaches was also modelled and analysed with an variance accounted for of almost 90%. The parameters obtained with respect to denaturation of the enzyme were highly comparable.