The separation performance of hydrogen sulfide-methane mixture by a nanoporous carbon membrane called selective surface flow (SSF) membrane is described. The membrane selectively permeates H2S from CH4, producing a CH4 enriched gas at the feed pressure. Experimental data for the separation of H2S-CH4 mixtures using feed-gas H2S compositions of 10-50 mol.% and feed-gas pressures of 0.45-1.14 MPa are reported. The partial pressure of H2S in the feed gas is the key Variable for establishing the separation performance of the SSF membrane. The H2S rejection-CH4 recovery data can be empirically correlated using a simple equation with a single adjustable parameter which is a function of feed-gas H2S partial pressure. The membrane separates H2S-CH4 mixtures very efficiently when the partial pressure of H2S is moderate (>0.2 MPa) even if the total feed-gas pressure is low (0.45 MPa).