The "Equilibrium Model" has provided new tools for describing and investigating enzyme thermal adaptation. It has been shown that the effect of temperature on enzyme activity is not only governed by Delta G double dagger(cat) and Delta G double dagger(inact) but also by two new intrinsic parameters, Delta H-eq and T-eq, which describe the enthalpy and midpoint, respectively, of a reversible equilibrium between active and inactive ( but not denatured) forms of enzyme. Twenty-one enzymes from organisms with a wide range of growth temperatures were characterized using the Equilibrium Model. Statistical analysis indicates that Teq is a better predictor of growth temperature than enzyme stability (Delta G double dagger(inact)). As expected from the Equilibrium Model, Delta H-eq correlates with catalytic temperature tolerance of enzymes and thus can be declared the first intrinsic and quantitative measure of enzyme eurythermalism. Other findings shed light on the evolution of psychrophilic and thermophilic enzymes. The findings suggest that the description of the Equilibrium Model of the effect of temperature on enzyme activity applies to all enzymes regardless of their temperature origins and that its associated parameters, Delta H-eq and T-eq, are intrinsic and necessary parameters for characterizing the thermal properties of enzymes and their temperature adaptation and evolution.