The rate of color formation in an activity assay consisting of phenol and hydrogen peroxide as substrates and 4-aminoantipyrine as chromogen is significantly influenced by hydrogen peroxide concentration due to its inhibitory effect on catalytic activity. A steady-state kinetic model describing the dependence of peroxidase activity on hydrogen peroxide concentration is presented. The model was tested for its application to soybean peroxidase (SBP) and horseradish peroxidase (HRP) reactions based on experimental data which were measured using simple spectrophotometric techniques. The model successfully describes the dependence of enzyme activity for SBP and HRP over a wide range of hydrogen peroxide concentrations. Model parameters may be used to compare the rate of substrate utilization for different peroxidases as well as their susceptibility to compound III formation. The model indicates that SBP tends to form more compound III and is catalytically slower than HRP during the oxidation of phenol.