New gas sensing devices based on solid state electrochemistry with a planar geometry are presented. The complete devices combine an improved processing technology of yttria-stabilized zirconia (YSZ) foils with platinum and semiconducting oxide electrodes. The microstructural and catalytic properties of the electrodes defined the sensing response, which exhibited a nonNernstian electrochemical nature, as expected. Sensors with WO/sub 3/-based electrodes showed the best sensitivity to NO/sub x/ among other options, with the highest selectivities in respect to CO at the lowest temperatures. In the case of Al/sub 2/O/sub 3/:Pt sensing electrodes, a differential gas diffusion principle of operation was considered, detecting CO without much selectivity to NO/sub x/.