Surface reactions of molybdenum-covered glass plates, used as back contact for thin Cu(In,Ga)(S,Se)(2) solar cells, are examined at open circuit in acidic electrolytes used for plating CuInSe2 layers. A multinary element cementation reaction occurs, leading to the corrosion of molybdenum and the formation of CuSex electroless deposit (x from 0.2 to 0.8), with well-defined compositions. Immediately after immersion, small nuclei appear on the surface, which then grow to form a quasi-continuous layer. The time evolution of the open-circuit potential of Mo and the Se/Cu surface atomic ratio are correlated and are in agreement with thermodynamic data predictions. (c) 2006 The Electrochemical Society.