The electrical characteristics of liquid-phase-deposited titanium oxide films on p-type (100) gallium arsenide substrate with ammonium sulfide treatment were investigated. The aqueous solution of hexafluorotitanic acid and boric acid was used as the growth solution for titanium oxide films. For the passivation of GaAs substrate surfaces with ammonium sulfide treatment, the electrical characteristics were much improved to that without ammonium sulfide treatment. The leakage currents were 1.04 and 1.91 X 10(-7) A/cm(2) under positive and negative electric fields at 0.5 MV/ cm. The dielectric constant and the effective oxide charges were 48 and 1.22 X 10(11) C/cm(2), respectively. The interface state density was 3.2 X 10(11) cm(-2) eV(-1) at the energy of 0.71 eV from the edge of the valence band. (c) 2006 The Electrochemical Society.