The study deals with information obtained from conventional isothermal inactivation experiments. When the lumped character of enzyme activity was analysed, identical activity-time profiles could be obtained from several mechanisms when the number of equivalent mechanisms increased exponentially with the number of parameters in the corresponding kinetic equations. The assessment of isothermal evaluation of inactivation kinetics was demonstrated using selected experiments from the literature exhibiting a deviation from first-order kinetics. Experiments were classified, according to the shape of the inactivation curve, into biphasic, grace-period and activation-period inactivations. A unified evaluation procedure, based on the discrimination among a few kinetic models derived from a general unimolecular series mechanism, was employed for all experiments. It was shown that regardless of the shape of the inactivation curve, only a few experiments needed more than a simple series mechanism represented by a biexponential curve, for a satisfactory fit. Carrying out inactivation experiments at several temperatures was shown to be very effective in checking the consistency of kinetic equations obtained by isothermal evaluation. The results of isothermal evaluation could always be questioned in this way and no consequent conclusions on inactivation mechanism could be drawn.