Four new poly(imide siloxane)s copolymers have been prepared by the reaction of 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA) with commercially available 4,4'-oxydianiline (ODA) and amino-propyl terminated polydimethylsiloxane (APPS). The copolymers were synthesized with variation of the APPS loading of 5, 10, 15 and 20 wt% (2.20, 4.55. 7.07 and 9.85 mol%), respectively. The poly(imide siloxane)s have been well characterized by different spectroscopic, thermal, mechanical, surface and gas transport techniques. The synthesized polymers exhibit good solubility in different organic solvents. The polymers with 5, 10,15 and 20 wt% siloxane loading show very good thermal stability in the range of 464-410 degrees C for 5% weight loss in synthetic air and glass transition temperatures in the range of 201-169 degrees C. All poly(imide siloxane)s form tough, transparent, and hydrophobic films, with tensile strength up to 65 MPa, a modulus of elasticity up to 1.42 GPa, elongation at break up to 25% and showing water contact angles up to 102 degrees. Thermal, mechanical, surface and gas transport properties of these polymers have been evaluated and compared with the homopolyimide without siloxane moiety. The permeability, diffusion and solubility coefficients for the gases as well as the ideal separation factors for gas pairs have been determined. The poly(imide siloxane) with 20 wt% (9.85 mol%) siloxane shows the best compromise with very high permeability and a still high permselectivity for the CO2/CH4 gas pair. (C) 2010 Elsevier B.V. All rights reserved.