Ultrathin graphene oxide (GO) (<5 nm) membranes were prepared by spin-casting onto microporous polymeric support membranes. GO membranes exhibited a highly CO2 permeable character, which is suitable for CO2 separation. In the presence of water vapour, high CO2 selectivity (e.g., CO2/H2, CO2/N2, and CO2/CH4) was achieved by enhanced CO2 sorption.