The miscibility of poly(methyl methacrylate) (PMMA) with polycarbonate (PC) polymer has been extensively studied, and emphasis was placed on the preparation conditions and the composition effects. The effect of added metal salt on the miscibility and the gas permeability of the PMMA/PC blend membrane was investigated. Trends of gas permeabilities and selectivities for PMMA/PC-rich blend membranes are similar to the miscible blend membranes, although the former membranes are translucent. The added metal salt complex modification effectively improved the miscibility of the PMMA/PC-rich blend membrane as evidenced from dynamic mechanical analysis and differential scanning calorimetry studies. The gas permeabilities of the added metal salt complexes of PMMA/PC-rich and PMMA-rich/PC blend membranes are higher than those of the corresponding noncomplexing blend membranes. Wide-angle x-ray diffraction analysis was made to study the difference of crystallinity for the effect of blend miscibility and salt additivity on PMMA/PC blend membranes.