Trans-polyacetylene, (CH)_x, can be p-doped (oxidized) to the metallic regime by gaseous oxygen in the presence of an aqueous solution of a non-oxidizing acid such as HBF₄. The oxygen oxidizes the (CH)_x to (CH^y+)_x while the HBF₄ supplies a stable fluoroborate counter-anion to give a species such as [CH^y+A^–_y]_x(y≈ 0.02; A^–= fluoroborate). The doping process is consistent with the reduction potentials of (CH)_x and O₂ in acid solution. In the absence of the acid, oxygen reacts irreversibly with the semiconducting (CH)_x to destroy its conductivity. The [CH^y+A^–_y]_x formed above may be connected to a lead anode in a HBF₄ electrolyte whereupon it is reduced to (CH)_x with concomitant production of an electric current. If oxygen is bubbled over the (CH)_x during this process it is continuously electrochemically reduced by the lead as rapidly as it is oxidized chemically by the oxygen. The (CH)_x therefore acts as a catalyst electrode: a ‘fuel-cell’ electrode for the room-temperature reduction of O₂ to H₂O at atmospheric pressure. The overall reaction does not occur to any significant extent in the absence of the (CH)_x.