It is discovered in this work that Cu(II) can be spontaneously reduced on titanium oxides in a mixed sulfate-chloride solution at 85 degrees C. X-ray photoelectron spectroscopy (XPS) suggests that after reaction, copper species exist in an oxidation state lower than 2+. Analysis using X-ray powder diffraction (XRD) shows that the reduced copper species could exist in the form of a CuTi alloy. Time of flight secondary ion mass spectrometry (ToF-SIMS) revealed the depth profile of copper signals in the oxide films. ToF-SIMS results demonstrated that copper signals co-existed with titanium oxide signals in the anodized oxide films, and copper species can diffuse deeply into the porous films within 60 minutes of immersion. Further electrochemical and surface characterizations have confirmed that the presence of chloride ions is a crucial factor for the spontaneous reduction of Cu(II). This phenomenon reveals the role of Cu(II) in a mixed sulfate-chloride solution and its effect on titanium oxide films. It also provides an approach to incorporate a CuTi alloy into titanium oxide films. (C) 2016 The Electrochemical Society. All rights reserved.