Soybean peroxidase (SBP) has an extremely high melting temperature of 90.5 degrees C at pH 8.0 in the presence of 1 mM CaCl2. The enzyme is substantially more thermostable than the peroxidases from horseradish (HRP) and Coprinus cinereus (CIP). SBP denaturation does not precisely fit the two-state denaturation model due to the formation of the apoenzyme upon initial melting. A pseudo-two-state denaturation can be assumed, however, and this gives rise to apparent kinetics for irreversible inactivation. The apparent kinetics indicate that irreversible deactivation is comprised primarily of enthalpic contributions, with Delta H-deact(double dagger) = 22.4 kcal/mol and T Delta S-deact(double dagger) = 0.2 kcal/mol at 95 degrees C. Heme transfer studies from the peroxidases to apomyoglobin indicate that SBP holds onto its heme much more tightly than does HRP, and this is consistent with a thermodynamically more stable enzyme.