A quantitative model is presented for the heat inactivation of enzymes at low or ultra-high temperatures. As a test of its validity, the heat inactivation of crude or partially purified lipase from Pseudomonas fluorescens, strain P38 (P38 lipase) was investigated. The activation energy (ΔE#), enthalpy (ΔH#), entropy (ΔS#) and Gibbs free energy change (ΔG#) for lipase inactivation at 40–140°C were determined. ΔH# for P38 lipase heat inactivation at 40–60°C, 50–80°C and 90–140°C was 170–221 kJ mol−1, −3 − −20 kJ mol–1 and 44–78 kJ mol−1 respectively. Over these temperature intervals ΔS# was 202–380 J mol−1 K−1, −318 − −375 J mol−1 K−1 and −92 − −186 J mol−1 K−1. ΔG# was 100–115 kJ mol−1 for enzyme inactivation at 40–140°C. The results are consistent with different rate-limiting reactions for P38 lipase heat inactivation at low or ultra-high temperatures. Within a narrow range of (intermediate) temperature, a third rate-limiting reaction may lead to ‘low-temperature inactivation’ phenomena. There was qualitative agreement between experimental results and the current model for enzyme heat inactivation.