Cylindrical internal reflectance infrared spectroscopy (CIR-FTIR) was used for the determination of the transport properties of hydrocarbons in zeolite L at high temperatures and pressures. This was the first time that the intracrystalline diffusivity of hydrocarbons in microporous media could be determined under supercritical conditions by using a dynamic technique. The study investigated the self-diffusion of n-heptane, the counterdiffusion of n-hexane and 1-hexene, and the codiffusion of n-heptane and p-xylene in zeolite L under subcritical and supercritical conditions. The diffusion time constants obtained for the single and binary systems showed that the diffusion within the pore was reduced significantly under supercritical conditions (i.e., liquidlike behavior) as the hydrocarbon density within the pore was increased.