Abstract: The effect of irreversible adsorption of cyclohexane, ethylbenzene, ortho-xylene and n-octane on diffusion in ZSM-5 molecular sieve was investigated by gravimetric method. For the above system, the ratios of diffusivities between two cases of with and without irreversible adsorption are: cyclohexane: 3.6(348K), ethylbenzene: 4.8(398K), 1.6(423K), ortho-xylene: 2.6(423K), n-octane: 12.3(398K). It shows that the diffusivity is increased by irreversible adsorption. This effect can be explained by the covering of the strong adsorptive sites with irreversible adsorbates, which decreases diffusional resistance and increases diffusivity.