Seed coat soybean peroxidase (SBP) belongs to class III of the plant peroxidase superfamily that includes the classical peroxidase, namely horseradish peroxidase (HRP). We have measured the catalytic activity (k c a t ) and catalytic efficiency (k c a t /K M ) of SBP and that of HRP-C for the oxidation of ABTS [2,2'-azino-bis-(3-ethylbenzthiazoline-6-sulphonate)] by hydrogen peroxide at 25 o C. We observed that the k c a t and k c a t /K M values for SBP are much higher than those for HRP-C at all pH values, rendering SBP a more potent peroxidase. This is attributed to the relatively more solvent exposed δ-meso heme edge in SBP. We observed that the maximum catalytic activity and conformational stability of SBP is at pH ~5.5. A pH maximum of 5.0 for the catalytic activity of SBP has recently been reported. Estimation of secondary structural elements at various pH values indicated that there is a maximal reduction of β-strands and β-turns at pH 5.5 causing the heme to be further exposed to the solvent and increasing the overall conformational flexibility of the protein.