Abstract: An improved membrane cap assembly is provided for polaragraphic oxygen sensors in which a preassembled cap is provided with a membrane mounted for receiving the electrodes thereagainst when the assembly is mounted on the sensor body in such a way that the secured portions of the membrane may slide or move in the secured state to accommodate or receive the electrodes. This has the effect of preventing destructive tension forces against the fragile membrane. Also, because of this arrangement, a more effective quantity of electrolyte is retained by the membrane adjacent the electrodes. This, in turn, has the effect of increasing the life of the assembly prior to electrolyte depolarization, and reducing the times necessary to dismantle and replace the assembly with a new cap, and a fresh quantity of electrolyte.