Poly(methyl methacrylate) (PMMA) was cast from five different solvents to form five membranes of different free volumes. It was found that the free volumes of the membranes followed the order of the boiling points of the solvents from which they were cast. An increase in FFV increased the gas permeability but apparently did not increase the gas solubility, The results indicated that the gas diffusivity in the membrane was strongly affected by the fractional free volume of the membrane, but that the gas solubility was not.