The thermal inactivation and storage behaviour for horseradish and spinach peroxidases were investigated in defined systems, in spinach also within its natural environment. The inactivation curves of either enzyme show a sharp bend which is clearly visible at low, but not at higher temperatures. The D-values were taken from the inactivation curves. z-values resulting from the D-values were 25.5 degrees C for horseradish peroxidase, 13 degrees C for isolated peroxidase and 18 degrees C for peroxidase in spinach extract. Horseradish peroxidase was relatively heat-resistent at pH 6.0, spinach peroxidase at pH 5.0-6.0; both enzymes were found to be highly susceptible to heat at pH 4.0. Peroxidase isolated from spinach responded differently to heating than the enzyme in spinach extract or suspension. This discrepancy indicates that certain model experiments cannot be transferred to foods. Heated peroxidase from horseradish and spinach were found to regenerate during storage; the extent of regeneration depended on the pH.