Fast adsorption-desorption kinetics with dead times of 25–35 ms have been followed using a new technique. At sorption equilibrium, a zeolite sample is subjected to small (less than ±1%) pressure changes brought about by very rapid, square-wave volume perturbations. By recording the gas pressure response on compression/expansion of the system, sorption uptake/desorption processes, which are governed by intracrystalline diffusion, can be followed. By fitting appropriate solutions of Fick's diffusion law to the adsorption/desorption curves, and correcting the as-measured diffusivities by the Darken factor, intracrystalline diffusion coefficients of short-chain hydrocarbons, n-hexane and benzene, have been determined. These corrected diffusion coefficients for methane, ethane, and propane, are ⩽ 5 smaller than the self-diffusivities measured directly by the n.m.r. pulsed-field-gradient technique. The corresponding coefficients for n-hexane and benzene agree well with those determined by piezometric measurements.