The fabrication of a heterojunction between p-doped polypyrrole and p-doped silicon and its possible use in the detecting of nitrogen oxide gas (NOx) is described. Pyrrole was photoelectrochemically polymerized onto a silicon surface from an aqueous solution in the presence of sodium p-toluenesulfonate or p-toluenesulfonic acid. The silicon surface was chemically treated in different ways. Current-voltage characteristics with regard to the viability of the development as sensor for NOx at room temperature were measured. At low forward bias voltages, the forward currents show standard diode behaviors with diode quality factors close to unity. The saturation current densities are of the order of 10(-8) A cm(-2). In the high forward bias voltage region, the heterojunction diodes exhibit a significant, fast and reversible response to NOx gas, described by a NOx-induced voltage shift, which corresponds to a NOx-induced decrease in the resistance of the polymer layer. Furthermore, it was found that the electrical properties of these heterojunction diodes depend strongly on both the supporting electrolyte used for doping and the chemical treatment of the silicon surface.