Sorption properties of silicalite-1 samples of pure silica form for some parafinic and aromatic hydrocarbons of different molecular sizes were investigated. It was observed that sorption rates and apparent equilibrium capacities for critically sized molecules, m-xylene and o-xylene, depend on the sorption history of the sample. Sorption of m- and o-xylene proceeded extremely slowly on fresh samples of silicalite-1, whereas sorption rates of these molecules on silicalite-1 samples that had passed through a previous p-xylene sorption/regeneration step were significantly faster and equilibrium capacities reached about 92-95% of sorption volume of silicalite-1 when sufficient time was allowed. Uptake curves for m- and o-xylene were of the sigmoid-type, indicative of lattice distortions of silicalite framework. The results of this work, along with some recent, independent studies involving n.m.r. MAS and XRD methods strongly suggest that sorption of some critically sized molecules causes subtle lattice distortions of high-silica ZSM-5 zeolites, modifying their sorptive behavior.