Microcanonical rate constants can be parameterized in terms of an effective temperature. We show here that for fairly general level densities of the involved species it is possible to make this parametrization with a temperature-independent heat capacity and a rate constant of the Arrhenius form. This is a major simplification in the interpretation of many experiments on unimolecular decay in vacuum. The results show that activation energies and heat capacities measured in microcanonical experiment differ from their canonical analogues. (C) 2014 Elsevier B.V. All rights reserved.