Acid phosphatase thermal deactivation follows a complex path: an initial decay toward an equilibrium distribution of at least two intermediate structures, mutually at the equilibrium, followed by a final breakdown toward a completely inactive enzyme configuration. The results obtained in the presence of sorbitol have been compared to those produced in the course of purely thermal deactivation of the native enzyme. For any sobitol concentration, an equivalent temperature is calculated that results in exactly the same activity-versus-time profile. This suggests enzyme deactivation to be controlled by a single, unchanging step. Immobilized enzyme runs have been performed, as well, by entrapping acid phosphates within a polymeric network formed onto the upstream surface of an ultrafiltration membrane. The stabilizing effect of entrapment cumulates with that produced by sorbitol. In this case, however, an equivalent temperature cannot be determined, thus indicating that a different deactivation mechanism is followed.