Measurements have been made of the permeability of nitrous oxide (N2O), carbon dioxide (CO2) and oxygen (O2) through silicone rubber and cellulose acetate membranes using a flat sheet permeator. Measurements have also been made of individual gas permeabilities from a mixed feed stream. N2O had the highest permeability when using the silicone rubber membrane, followed by CO2, with O2 an order of magnitude less. All three gas permeability coefficients were independent of feed pressure for this membrane and N2O and CO2 permeability coefficients were temperature independent. For the cellulose acetate membrane, N2O and CO2 permeabilities varied with feed pressure and all three gases gave positive temperature coefficients. No separation of CO2 from N2O could be achieved under any conditions with the cellulose acetate membrane.