Electrolytic Li3PO4 coating on Pt as the solid electrolyte has been carried out in the mixture of LiNO3 and NH4H2PO4 aqueous solution. The cathodic reactions during the polarization could be divided into three stages: (I)O-2 + 2H(+) + 2e(-) -> 2OH(-) (+0.49 to -0.33 V); (II) 2H(+) + 2e(-) -> H-2 (-0.33 to -0.68 V); (III) 2H(2)O + 2e(-) -> H-2 + 2OH(-), HPO42- + OH- -> PO43- + H2O and H2PO4- + 2e(-) -> PO43- + H-2 (-0.68 to -2.0 V). The orthorhombic beta-phase Li3PO4 was found until the transition to orthorhombic gamma-phase at 500 degrees C. The lithium ionic conductivity of 100 nm crystalline Li3PO4 thin film was about 8.62 x 10(-8) S cm(-1) at 25 degrees C. Also, the mechanism of electrolytic Li3PO4 coating on Pt was discussed in this article. (c) 2006 Elsevier B.V. All rights reserved.