Isotopic-transient permeation measurements were performed to measure the diffusion coefficients in rubbery polymer membranes during steady-state pervaporation (PV). The transient permeation was done by adding a mixture containing a deuteride to the feed mixture after the permeation for the original feed mixture reached steady state. The additional mixture had the chemical composition equivalent to that of the original feed mixture. The transient permeation was followed by means of mass spectroscopy. Pervaporation of pure water through a polydimethylsiloxane (PDMS) membrane and binary liquid mixtures of benzene (Bz) and cyclohexane (Cx) through crosslinked poly(ethylene oxide imide) segmented copolymer (PEO-PI) membranes was examined. The time lags measured for these systems ranged from 9 to 430 s. The apparent diffusion coefficients D-a were determined from the time lags. The D-a values for Bz and Cx in PEO-PI membranes increased with an increase in the total concentration of the penetrants sorbed in the membranes. The local diffusion coefficient D(C) of each component was evaluated from the concentration dependence of D-a. The D(C) was used to calculate the concentration-averaged diffusion coefficient, which was in agreement with value obtained by the conventional sorption and PV experiments to binary liquid mixtures. The isotopic-transient permeation experiment can be used to determine the D(C) for a membrane under steady-state PV operation.