The oxidation state of the superconducting oxide, La2CuOy, oxidized by electrolysis, which shows higher superconduction, was determined by a wet-chemical procedure based on flow-coulometry. The ratio of CuO+/Cu-total in the bulk sample (X(b)) after electrolysis depends strongly on the potential and the electrolysis time, and the reducibility of X(b) in the sample after re-electrolysis was observed. The electrolytic reactions of La2CuOy with the solution were estimated as CuO+ + 2H(+) + e(-) reversible arrow Cu2+ + H2O in acid and CuO+ + H2O + e(-) reversible arrow Cu2+ + 2OH(-) in alkali solution, accompanied with hydrogen or oxygen evolution. The superconductivities in these samples were measured, and it was confirmed that the superconducting property in La2CuOy increases with X(b).