The morphology of cellulose acetate/poly(methyl methacrylate), CA/PMMA, blends of various compositions was studied. Optical microscopy and DSC conclusively showed that the polymers form incompatible blends. However, partial miscibility between the phases was noted since the glass transition temperature of the PMMA phase increased with the increase in the amount of the CA fraction in the blend, while the glass transition temperature of CA decreased with the increase in the amount of the PMMA fraction in the blend. ATR-FTIR spectra and surface energy measurements showed that the CA/PMMA film surfaces were comprised primarily of PMMA polymer. These results suggest that the CA/PMMA blends form a layered morphology with surface layers comprised essentially of PMMA polymer and the interior layer comprised of phase-separated blend. He, O2 and N2 gas transport properties of CA, PMMA and CA/PMMA blends of several compositions have been measured at 35-degrees-C. The data were analyzed in terms of parallel and series resistance models. The high He/O2 and He/N2 gas separation factors were best described by a combination of the parallel and series models that is consistent with the proposed layered film morphology. A novel composite gas separation membrane was prepared by coating porous polysulfone hollow fibers with a solution of the CA/PMMA blend. CA/PMMA composite hollow fiber membranes with a broad range of coating thicknesses were prepared and their He and N2 gas transport properties were measured. The properties were compared to the gas transport properties of the CA and PMMA composite hollow fiber membranes of equivalent coating thicknesses. The CA/PMMA composite membranes exhibited a superior combination of He/N2 gas separation factors and He permeation rates compared to the CA or PMMA composites. The superior properties were attributed to the unique layered morphology of the coating comprised of PMMA exterior layer and mostly CA interior layer. The formation of layered morphology was supported by surface energy measurements. The surface energy of the CA/PMMA composite hollow fiber membrane was measured and found to be essentially identical to that of the PMMA polymer. The multilayer coating morphology was observed directly under SEM examination of the hollow fiber cross sections. CA/PMMA composite membranes can be used to purify helium gas streams to produce a high purity product (less than 10 ppm contaminant level) with high helium recovery in a single-stage process.