The electrochemistry of solid polymer electrolyte-based amperometric devices, fabricated according to a recently introduced planar design concept, was further investigated. The solid protonic conductors used were Nafion®-117 membranes and polybenzimidazole (PBI) films while the working and counter electrodes were Au layers sputtered on the same face of the solid electrolyte and in contact with the gas sample. The test gas was oxygen, and sensors based on both materials exhibited linear response to oxygen concentration changes and response times comparable to those of commercially available sensors. The shape of the oxygen reduction current–potential curves observed point to high mass transport rates and the absence of a significant mass transport barrier. A systematic study of the effect of humidity on oxygen reduction and Au surface electrochemistry at the Au/Nafion® and Au/PBI interface reveals that the fall in the oxygen signal with decreasing humidity is not merely due to ohmic losses.