Chalcogenide semiconductors, such as CuInSe2, are promising materials for optoelectronic applications such as solar cells. Conversion efficiencies up to 14% based on Cu-In-Se layers electrodeposited have been achieved. However, the electrocrystallization of this multinary system, whose components exhibit different electrochemical and chemical properties, is a complex process. In the present work, the electrocrystallization mechanism of CuInSe2 is investigated in dilute acidic solutions by in situ electrochemical investigation (polarization, electrochemical quartz crystal microbalance, and impedance spectroscopy) and ex situ chemical and structural analyses. A detailed X-ray photoelectron spectroscopy analysis is carried out to determine the surface composition and the chemical environment of the elements. Marked differences between surface and bulk concentrations are evidenced. At low polarizations, binary Cu-Se phases are obtained, which facilitate the reduction of Se(IV) species into elemental selenium. An intermediate CuSe2 phase is formed which, in turn, enables the incorporation of indium beyond a potential threshold close to -0.6 V/mercurous sulfate electrode. In this potential zone, major transitions are observed in composition, morphology, structure, and electrochemical behavior. A reaction path is proposed, which accounts for the main experimental observations. It emphasizes the role of surface reactions, occurring via passivating and nucleation sites, in the incorporation of indium. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3374590] All rights reserved.