CO2, CH4 and N-2 adsorption properties of KFI- and RHO-polyvinyl acetate mixed matrix membranes (MMMs) were determined. A comparison was made between experimental and calculated adsorption capacities of the membranes. The fractal dimensions of the membranes were calculated from CO2 adsorption data. A common discrepancy existed between experimental and calculated adsorption capacities of MMMs, which may be rationalized by the presence of an interphase between filler and polymer phases. The higher fractal dimension of KFI-polyvinyl acetate MMM, compared to RHO-polyvinyl acetate MMM indicated higher size sensitivity and thus an additional advantage for the adsorption of smaller molecules, such as CO2.