The carbon membrane containing Fe-nitrate has promising separation performance for the gas pairs O2/N2 and CO2/CH4. Carbon containing nitrates of Cu or Ag show high selectivity, but reduced O2 and CO2 permeability compared to carbon with Fe-nitrate. Element analysis indicates that Cu migrates to the carbon surface, creating an extra layer resistance to gas transport. A silver mirror is also seen on the surface of Ag-nitrate-containing carbon. However, the Ag- and Cu-containing membranes show a high H2 permeability. Adding metal oxides makes the carbon membranes retard the transport of easily condensable gases (e.g. CO2). This can be exploited for enhanced H2/CO2 separation efficiency.