A miniaturized gas sensor comprised of thick- or thin-film type electrodes, on a non-conductive supportive substrate, and in contact with a solid ionomer electrolyte, for the detection of toxic gases, i.e., carbon monoxide, and other oxidizable or reducible gases and vapors is described. The all-solid planar sensor cell has two or more film type electrodes arranged on a non-conductive planar surface of a supportive substrate. The electrodes are discrete and in intimate contact with the same solid polymer ionomer membrane. The sensor cell contains no liquid electrolyte and is operated in a constant-voltage, potentiostatic or potentiodynamic mode. A high sensitivity to a select gas or vapor is achieved by a novel three-phase contact area design for a sensing electrode which provides contact with the solid ionomer electrolyte, as well as the gas sample via diffusion openings or holes that penetrate through the supportive substrate.