Abstract: A planar gas sensor, in particular a pCO.sub.2 or pO.sub.2 sensor, has the following components: a pH sensor located on a flat substrate, or at least one noble metal electrode located on the substrate; a reference electrode adjacent to the pH sensor or noble metal electrode; an electrolyte layer, covering the reference electrode and the pH sensor or noble metal electrode, which is delimited by a first polymer structure; and a layer of hydrophobic material, covering the electrolyte layer and a surface region of the substrate outside the first polymer structure, which is delimited by a second polymer structure.