In situ high-temperature X-ray diffraction was used to investigate the reaction mechanism and kinetics of alpha-CuInSe2 formation from Cu-In precursors during selenization. The precursor films were deposited in a migration-enhanced molecular beam epitaxial reactor on Mo-coated thin glass substrates. During the selenization, the formation of CuSe was observed, followed by its transformation to CuSe2 at higher temperature. The formation of alpha-CuInSe2 was initiated at a temperature between 250 and 300 degrees C. Additionally, the production of MoSe2 was clearly detected at temperatures above 440 degrees C. The reaction kinetics were analyzed using both the Avrami and parabolic rate models to estimate diffusion-limited activation energies of 124 (+/- 19) and 100 (+/- 14) kJ/mol, respectively. (c) 2006 Elsevier B.V. All rights reserved.