A planar polarographic sensor for determining the lambda value of gas mixtures, for use with exhaust gases in internal combustion engines, includes a planar ceramic carrier sheet having an outer pumping electrode and an inner pumping electrode respectively arranged on opposite planar sides thereof, and a diffusion barrier. The outer and inner pumping electrodes, and the diffusion barrier are arranged on the planar ceramic carrier sheet so that the measuring gas is fed via the diffusion barrier to the inner pumping electrode. The sensor has front and side edge faces. The diffusion barrier is configured as a diffusion layer at least partially covering the inner pumping electrode and exposed at least at one edge face of the sensor to the measuring gas. A diffusion-free planar region of the sensor is provided with an equalizing layer having the same thickness as the diffusion layer, and the equalizing layer is disposed in the same plane with and adjacent to the diffusion layer.