Gas transport properties in homogeneous blends of PMMA with each of two SAN random copolymers, containing 13.5 and 28% by weight of acrylonitrile respectively, have been measured a t 35OC for He, H,, O,, N,, Ar, CH,, and CO,. For all cases, the permeability and diffusion coefficients are higher than that expected from the semilogarithmic additivity rule. On the other hand, the solubility coefficients and the ideal gas separation factors follow this rule well. These results for PMMA/SAN blends differ from those observed recently for other miscible blend systems; however, they agree well with recent theories proposed to describe gas sorption and permeation behavior in polymer mixtures. The compwition dependence of gas transport properties observed in PMMA/SAN blends is attributed to the very weak net interactions between PMMA and SAN produced by repulsions between styrene and acrylonitrile units in the SAN random copolymers. Gas transport properties in phase-separated PMMA/SAN blends have also been studied. The phase-separated blends show sorption and permeation properties very similar to the corresponding homogeneous blends which can be explained by an isotropic, interconnected, two-phase model pro@ by Kraus and Rollmann. Gas permeabilities for the solution cast PMMA films used here are compared with melt-extruded specimens used previously, and the differences are attributed to molecular orientation.