Intracrystalline diffusivities of Y type zeolite were measured in the temperature range of 323-573 K for four aromatics : benzene, toluene, and ortho-, and para-xylene, and four paraffins : n-hexane, n-heptane, n-octane and iso-octane. Three model equations for calculating the intracrystaline diffusivity were derived each based on one of the factors thought to control the diffusion : the probability for a molecule to jump from a supercage to a neighbouring supercage through a narrow window. the Lennard Jones (12.6)-potential induced by the van der Waals attraction and the electrostatic field formed in a supercage. The measured intracrystalline diffusivity was found to be well represented by the model equations based on the electrostatic field mechanism.