A detailed structural analysis of the (AgCu)(InGa)Se-2 thin film alloy system was undertaken via X-ray diffraction in order to determine its phase behavior and the chalcopyrite phase lattice constants of the alloy system. Thin films were grown by elemental co-evaporation with time-invariant flux, for the compositions 0 <= [Ag]/([Cu] + [Ag]) <= 1 for fixed [Ga]/([In] + [Ga]) = 0.5. Lattice constants were determined from the diffraction patterns by the Cohen method and were found to deviate from Vegard's rule. While films were predominantly single-phase, minor secondary phase reflections were observed for films with [Ag]/([Cu] + [Ag]) >= 0.5. However, this secondary phase behavior is not consistent with chalcopyrite-chalcopyrite phase segregation in earlier reports. (C) 2011 Elsevier B.V. All rights reserved.