Assay results for a single sample stored in an elevated temperature oven for a known period of time may be in error if instantaneous heating and cooling of the sample is not achieved. For usual kinetic studies however, the storage behavior of a group of samples which have been placed in the oven at the same time is followed. Theoretical and experimental considerations are presented to show that in the case of apparent first-order reactions the assay values for all samples in the group are subjected to the same degree of error. Rates of degradation calculated from the assay data are identical to those measured under isothermal conditions. Consequently, decomposition rates obtained under the usual oven storage conditions, where time lags in heating and cooling samples exist, give useful information for estimating product stability.