In this study, the behavior of enzyme activity as a function of pH and temperature is modeled on the basis of fundamental considerations. A formulation is developed that includes the activation of enzymes with increasing temperatures and the deactivation of enzymes at higher temperature, together with the effect of protonation and hydroxylation on activity at various constant pH levels. The model is calibrated and validated against an extensive set of experimental data on phytases from seven different origins. The percentage variance accounted for (R-adj(2)), obtained by statistical nonlinear regression analysis on all data sets, was shown to range from 97.6% to 99.5%. The equilibrium constant of protonation and hydroxylation proved to be independent of temperature. (C) 2001 John Wiley & Sons, Inc.