A very simple method of constructing a differential pCO₂ sensor device is described, using polyaniline-based pH electrodes in place of the pH-ISFET-based system employed previously. The polyaniline film-coated Pt electrode was shown to exhibit not only an enhanced potentiometric pH response but also a greatly reduced oxygen sensitivity compared with the uncoated Pt electrode. The proposed differential system employs two identical polyaniline electrodes to make a pCO₂ probe and a reference electrode. Both electrodes are made by coating the polyaniline surface with a gas-permeable silicone rubber membrane doped with valinomycin. The pCO₂ electrode is covered with a hydrogel-based recipient layer inside the gas-permeable membrane whereas the reference electrode does not have the hydrogel layer. In this sensor configuration, the emf differences between the pCO₂ and the reference electrodes serve as analytical signals and hence the ion responses caused by the two parts of the outer gas-permeable membranes cancel out. The polyaniline-based gas sensor system was shown to provide better emf stability than the uncoated Pt electrode-based counterpart.