The surface composition of coevaporated Cu-In-S films for photovoltaic applications has been investigated by x-ray and ultraviolet photoelectron spectroscopy. A comparison of the front and back surface, i.e., the interface between the Cu-In-S film and the Mo back contact, is made for films with different stoichiometries. We find that In-rich films are covered at both surfaces by an In-rich second phase which has the cation ratio of CuIn3S5. The front surface of Cu-rich films shows a segregation of CuS that is not found at the back surface. The formation of binary and ternary second phases in nonstoichiometric CuInS2 films and their implications on the growth mechanism are discussed.