The effect of various diffusion barriers on the mass transport controlled current for an amperometric gas sensor with a porous membrane is considered. A general equation is derived which relates the total current to the diffusion parameters for each barrier. By varying the thickness of one barrier at a time, keeping all the other parameters constant, the relative roles of each of the barriers can be determined. This is done for CO and H₂S sensors and in both bases it is shown that more than 70% of the transport control is provided by the electrolyte film in the porous electrode/membrane. This conclusion is supported by observations of the temperature dependence of the sensor currents. It is concluded that, in order to obtain more consistent and reproducible results from porous membrane covered amperometric gas sensors, a more controllable method of electrode manufacture and assembly would be desirable.