Locke’s maximum slope method for the reduction of transient heat transfer test data is extended to include arbitrary upstream fluid temperature changes. The analytical solution of the single-blow problem is used to evaluate maximum slopes which are shown to depend, in general, nonmonotonically on the number of transfer units, Ntu. It is shown that there is a critical number of transfer units, (Ntu)crit, such that, for Ntu > (Ntu)crit, the maximum slope method remains applicable. In illustration of the analysis, maximum slopes and maximum slope errors are presented for various upstream temperature changes deviating from the step change.