Diffusion behavior of several organic solvents in polyisobutylene(PIB) and in poly(p-methylstyrene-co-iso-butylene)(PMS-BR) with different monomer ratios has been studied. The experiments have been conducted over a tem-perature range of 50 to 100℃ using a conventional gravimetric sorption technique. The PMS-BR copolymers contained 2, 7, and 15 weight percent p-methylstyrene, respectively. Although employing temperatures were far above the glass tran-sition temperatures of polymers, the diffusion coefficients are correlated well with the Vrentas-Duda free-volume theory. For all the solvents, the PIB shows the highest diffusivity while the copolymer with the 15% p-methylstyrene gives the lowest value. This behavior can be explained by the amount of fractional free-volume present in a system.