Mixed matrix composite membranes of a conducting polymer, polypyrrole (PPy), and an insulating polymer, polybisphenol-A-carbonate (PC) were prepared by a combined in-situ polymerization and solvent evaporation. Mixed matrix composite membranes were synthesized to combine the good gas transport properties of conductive polymer, PPy, with good mechanical properties of PC. The permeation rates of some commercially important gases. Ar, CH4, CO2, H-2, N-2 and O-2 were measured. Effects of different preparation parameters on the membrane performance were investigated. The parameters investigated were the effect of composition (in terms of PC content), effect of hydrophilicity or hydrophobicity of the surface, annealing, solvent evaporation temperature and supporting electrolyte type and its concentration on the membrane performance. The SEM images and TGA graphs have showed that, the most effective parameter on the membrane performance was the supporting electrolyte type and concentration. Best results were obtained for a membrane dried at 100 degreesC which is cast from an initial solution of 7% PC, containing 0.01 M para-toluene sulfonic acid (PTSX) as support electrolyte. Mixed matrix membranes of PPy with PC showed higher selectivities with respect to dense, homogenous, flat sheet PC membranes. Selectivity values for this membrane are CO2/N-2 = 5.9, O-2/N-2 = 4.2, H-2/CH4 = 5.2 while permeabilities for CO2, O-2, H-2 are 11.6, 8.2 and 19.5 Barrers, respectively.