Chymotrypsin was irreversibly inactivated at 30–130°C, pH 1.0–7.0 and in the presence of 0–4.0 m guanidine hydrochloride (GnHCl). The activation enthalpy for enzyme thermoinactivation (Δ#) at moderate temperatures, pH 4.0–7.0 and in ≤2 m GnHcl was 175–322 kJ mol−1. The activation entropy (ΔS#) was 244–734 J mol−1 K−1. Such results are compatible with enzyme unfolding being the rate-determining step for the thermoinactivation of native chymotrypsin. For chymotrypsin pre-unfolded at low pH, high temperature and/or in Gn/HCl, ΔH# was 30–40 kJ mol−1 and ΔS# was between −182 and −191 J mol−1 K−1. Therefore, thermoinactivation of pre-unfolded chymotrypsin is likely to involve covalent bond lysis as the rate-determining step. A biphasic Arrhenius plot was obtained for chymotrypsin thermoinactivation in 1.0–1.5 m GnHCl. Taken together, these results provide strong support for the two-stage model for enzyme thermoinactivation.