By the incorporation of filler materials in a membrane one may change the permeability of the overall, dense membrane towards Various gases, vapours and/or liquids. The influence of solid fillers dispersed in a polymeric matrix on permeability, diffusivity and solubility has been studied extensively in literature. In this paper the influence of the filler on overall membrane permeability resulting in an upper and a lower Limit of the permeability given the filler content and the permeability of the continuous phase will be studied theoretically. From this analysis the change in membrane selectivity towards a gas mixture by incorporation of a filler, relative to the selectivity of the pure matrix membrane, can be obtained. The maximum change in membrane selectivity is a function of the filler content only, i.e. irrespective of matrix and filler permeabilities. The difficulties often encountered in estimating the filler permeability from experimental permeability data using membranes with varying amounts of the filler will be discussed, and possible solutions will be given using theoretical and experimental data.