The diffusion and sorption of acetylene has been used to characterize the strength of the acid sites in various zeolites and their different cationic forms. The frequency-response (FR) technique has been employed in these studies. Rate spectra were recorded at temperatures between 0 and 100degreesC and in the pressure range of 50-500 Pa. The results obtained suggest that intracrystalline diffusion is the rate-controlling step if the zeolite channel diameters are close to the kinetic diameter of the alkyne. Because of the strong interaction of acetylene with the metal cations the resulting intracrystalline diffusivities were lower than those of propane which has a larger kinetic diameter. In the H-forms of medium and large-pore zeolites the rate of sorption on the acid sites governs the rate of transport because of the low diffusional resistances in the larger micropores. In different cationic forms of Y zeolite the FR sorption peaks associated with the various Lewis and Bronsted acid sites generated by the multivalent cations cannot yet be distinguished and assigned in contrast to IR spectroscopic results. With Na-X pellets the rate-determining step of the sorption process is diffusion through the intercrystalline channels indicating the importance of this mass transfer resistance in catalyst pellets,