Electrodeposition of multicomponent oxide films with semiconducting properties for use in various applications like photovoltaics is of great interest. This paper deals with mixed copper, indium, gallium oxides which can be used as precursors for copper indium gallium diselenide layer in thin film solar cells and potentially p-type transparent oxide layers, in the delafossite family. One-step cathodic electrodeposition of Cu-In-Ga mixed oxide/hydroxide is investigated using hydrogen peroxide as the oxygen source. The deposition mechanism of the Cu-In-Ga oxide layers is studied by cyclic voltammetry and in situ using an electrochemical quartz crystal microbalance. Amarked influence of hydrogen peroxide on the Cu(II), In(III) and Ga(III) electrochemical system is evidenced. Hydrogen peroxide enables the incorporation of indium and gallium as oxides/hydroxides at much less negative potentials than the In and Ga metals. A reaction path is proposed which accounts for the main experimental results. The reduction process starts with the reduction of cupric species, which is followed by the reduction of hydrogen peroxide. For peroxide concentrations higher than 20 mM, the precipitation of Ga and In oxides/hydroxides takes place. At more negative potentials the reactions become controlled by mass transfer allowing to control the composition of the films. (c) 2017 The Electrochemical Society. All rights reserved.