The objective of this study is to establish a mathematical model describing vapor diffusivity as a function of its activity and sorption level in a poly(dimethyl siloxane) (PDMS) membrane for multicomponent feed systems. The gravimetric method was used to determine the vapor diffusion coefficients. At time intervals, the sorbed membranes were heated and purged to analyze the contents of individual vapor components. The data of sorption amount as a function of time elapsed in the transient period were fitted for the diffusion coefficient using Fick's first law. The Long's model was used to fit the diffusivity as a function of sorbed vapor content. It was found that the vapor diffusion coefficient in a ternary system was similar to that in a binary system at the same sorption level.