AbstractPoly(imide-siloxane)s differing in their composition were prepared and characterized. The starting polymer (control) was the hyperbranched polyimide based on 4,4′-oxydiphthalic anhydride and 4,4′,4″-triaminotriphenylmethane. In the poly(imide-siloxane)s, 10 or 40–50 mol% of the 4,4′,4″-triaminotriphenylmethane was theoretically substituted for by amine-terminated siloxane dimer or oligomers. 1,3-Bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, bis(3-aminopropyl)-terminated poly(dimethylsiloxane) with a theoretical number-average molar mass of 1000 g·mol⁻¹ and bis(3-aminopropyl)-terminated poly(dimethylsiloxane) with a theoretical number-average molar mass of 2500 g·mol⁻¹ were used for this purpose. The thermo-mechanical properties and gas separation characteristics for hydrogen, carbon dioxide and methane of these polymeric products were shown to be dependent on their composition.