The variation of the permeation properties of membranes with respect to the effective sizes of the molecules employed is investigated. Data reported in the literature corresponding to ceramic and metal-supported silicalite membranes, polymeric Teflon membranes and silicalite filled PDMS mixed matrix membranes are employed in the calculations. Additionally, experiments are performed to obtain the permeabilities of the polymeric mixed matrix membranes filled with various ion-exchanged forms of the ZSM-5 zeolite. In general, quite linear relationships are obtained between the log (permeability) and the log (size) values. This implies that the membranes investigated in this study exhibit significant size scaling behavior For such membranes. a relatively higher degree of the size sensitivity signifies a relatively higher ability for discriminating between molecules of different sizes. This provides a unique opportunity for enhancing the selectivity values since selectivity for one component in a mixture of gas molecules of different effective sizes is related to the degree of the size sensitivity of membranes. Modification methods, such as ion-exchange, seem to have the potential of creating differences in the scaling properties of zeolite samples and thus providing higher selectivities. Increasing the loading of the zeolite in the zeolite-polymer mixed matrix membranes may also lead to a similar result in case the size sensitivity of the zeolite is relatively higher than that of the polymer. When the size scaling information is to be derived and applied for the purpose of monitoring the selectivities, attention should also be paid to the temperature and pressure ranges of applicability. (C) 2001 Elsevier Science B.V. All rights reserved.