Abstract: The present invention concerns gas-separation membranes. In some embodiments, the membranes comprise nano- or microislands or non-continuous metal layers suitable to adsorb, react with and/or otherwise retain at least one of the gases to be separated from a mixture of gases. In one embodiment, the membrane comprises a porous graphene membrane on which Pt or Pd nanoislands are deposited using a mesh, while the nanoislands are suitable to retain and bind to H2 while letting helium pass in a H2/He gas mixture. In another embodiment CO2 is separated from a H2/CO2 mixture. The membranes exhibit both, high selectivity and permeance and can be operated at room temperature.