Careful analysis of the dependence of enzyme activity on assay temperature has revealed that some enzymes might have real temperature optima in which the decrease in catalytic rate at temperatures above the optimum is not primarily a result of irreversible thermal inactivation. The ‘equilibrium model’ has been formulated to describe genuine temperature optima, and to suggest a simple experimental method by which to distinguish these cases from those in which enzyme instability is the major determinant of temperature optima.