Time-resolved FTIR microscopy was used to investigate in situ the transport and sorption of toluene in individual ZSM-5 crystals with varying degree of perfection. The results indicate that the rate of transport of toluene in the pore system of zeolite ZSM-5 strongly depends upon the degree of crystal intergrowth. The diffusion coefficients observed with single crystals were 3 orders of magnitude higher than the diffusion coefficients measured for a polycrystalline sample. The first results of this new method to study diffusion in microporous materials are compared to the results from other techniques.