Gas permeability through fixed site carrier membranes (FSCMs) is predicted by the RC circuit models. If only one permeability value is available, other permeability values can readily be estimated as a function of applied pressure for a FSCM with given backward reaction rate and reaction equilibrium constants between carrier and penetrant, carrier concentration, and matrix permeability. The results were compared with experimental oxygen permeabilities through a PMMA membrane containing metallophorphyrin, The agreement between them was exceptional. in addition, the backward reaction rate constant between carrier and penetrant and the pressure fluctuation are found to be most sensitive in determining permeability in FSCMs according to the sensitivity analysis,