The thermal deactivation of TEM-1 β-lactamase was examined using two experimental techniques: a series of isothermal batch assays and a single, continuous, non-isothermal assay in an enzyme membrane reactor (EMR). The isothermal batch-mode technique was coupled with the three-state "Equilibrium Model" of enzyme deactivation, while the results of the EMR experiment were fitted to a four-state "molten globule model". The two methods both led to the conclusions that the thermal deactivation of TEM-1 β-lactamase does not follow the Lumry-Eyring model and that the T(eq) of the enzyme (the point at which active and inactive states are present in equal amounts due to thermodynamic equilibrium) is at least 10 °C from the T(m) (melting temperature), contrary to the idea that the true temperature optimum of a biocatalyst is necessarily close to the melting temperature.