Diffusivity data for alkanes in zeolites, as reported in the literature, may vary by orders of magnitude. A satisfactory answer as to the nature of these large differences has not been presented as yet. Presently we have assessed the impact of noble gases on diffusion. This is relevant as in many cases diffusion rates are obtained in diluted systems. The results obtained may in part explain the above-mentioned discrepancies. It is generally believed that these gases are inert in the sense that their presence does not, to a significant extent, influence the diffusivities of other components. We have investigated the impact of helium and argon on the adsorption isotherms and on the adsorption rate of n-butane and isobutane in the zeolites FER, MFI, and BEA. We found a pronounced effect. The equilibrium times may increase by as much as a factor of 6, when admitting inert gases prior to the adsobate, whereas no such an effect was seen for the reversed order of admittance. Also an effect on the equilibrium was found.