The fabrication and working principle of an electrochemical gas sensor for direct gas phase detection of organic molecules is presented. The sensor is composed of two Pt electrodes where redox reactions can occur, and an ion exchange membrane to conduct H+ from an electrode to the other. As proton exchange membrane they used a spin-coated Nafion® layer. An underlying SU-8 layer assures a double role of adhesion of the Nafion to the substrate and water reservoir to limit dehydration of the Nafion. The sensor is characterised for its electrochemical properties and tested for the detection of CO as model pollutant. The detection tests are performed in the form of cyclic voltammetry, and show that the sensor can detect the gas at the applied voltage corresponding to the oxidation potential of CO into CO2.