In sensors with porous gas diffusion membranes, the major control of mass transport often occurs at the three phase boundary between the gas, the electrolyte and the electrocatalyst [1]. This interphase is poorly defined and difficult to control. An attempt has been made using sputter deposition to make a more reproducible and controlled working electrode-gas environment. The electrodes have been tested in commercial sensors for ppm levels of CO, H2S, SO2 and Cl-2. All sensors showed the unexpected result of two response maxima as a function of thickness of platinum deposited. The electrochemical area of the film also showed a similar two maxima relationship. Cross-sectional SEM has been used for the analysis of the pore size distribution and the sensor response time and electrode resistance have also been measured, also as a function of electrode thickness. A simple model is proposed to explain the observed relationship. From a practical viewpoint, there is evidence that sputter deposition does allow better control of electrode structure than more traditional methods of production.