Phases and phase transitions in three binary systems, Cu-Se, In-Se, Cu-In and in the ternary and quaternary systems Cu-In-Se (Ga) were investigated by in situ high energy powder diffraction in a temperature range from 25 to 550 degreesC. Results for the binary systems are compared to the known equilibrium phase diagrams of Cu-In, Cu-Se and In-Se. Above 225 degreesC Cu-In and Cu-Se follow the equilibrium phase diagrams. For In-Se significant deviations from the equilibrium diagram are observed. The results on binary systems yielded the basis for the qualitative phase analysis of the phase sequences observed in the CuInSe2. precursors during thermal anneal. On ternary and quaternary systems Cu-In-Se-(Ga) the reaction path to the formation of CuInSe2 could be deter-mined in real time. Deviations of phase sequences from the equilibrium phase diagrams are attributed to the digression from the chemical rather than the thermal equilibrium. CuInSe2 finally crystallises from the direct precursors Cu2Se (Cu2-xSe, respectively) and InSe within a melt rich in selenium. The influence of gallium and sodium on the phase sequences and the resulting formation of CuInSe2, are discussed. Both, Na and Ga promote the crystallisation of Cu2Se, the direct precursor phase for CuInSe2. A comparison of the crystallographic structures of Cu2Se and InSe shows that epitaxial growth of InSe (0001) on Cu2Se (111) lattice planes is feasible. Based upon the experimental and crystallographic analysis a qualitative model for CuInSe, crystallisation from the precursors in the melt is developed. (C) 2003 Elsevier Science B.V All rights reserved.