The cz,t (product concentration vs. time) curve is transformed into a Δcz,r curve, where Δcz is the difference in product concentration at the end and beginning of the rth time interval. If the duration of these intervals is an increasing function of time, the Δcz,r curve passes through a maximum. The theory of this function is developed for first-order reactions, analytical approaches are suggested, and the technique is applied to experimental data on three-component mixtures. © 1979, American Chemical Society. All rights reserved.