The phase separation in amorphous silicon suboxide (a-SiOx) films upon thermal annealing for the formation of light emitting silicon nanocrystals (Si-NCs) was studied through the correlation of photoluminescence (PL) and photoluminescence excitation (PLE) with structural and optical properties. The PL and PLE features and the structural and optical properties show a strong dependence on the annealing process and reveal that the precipitation of the excess Si in a-SiOx and the formation of Si-NCs from the precipitated Si are two separate processes which should be distinguished in the phase separation in a-SiOx. They proceed at different temperatures and the formation of Si-NCs is a slow process compared with the precipitation of the excess Si. The nanocrystal size and size distribution evolve with annealing time at the initial stages and are mainly dependent on annealing temperature for a certain O content in the initial a-SiOx with the density of the formed Si-NCs increasing with longer annealing duration. (C) 2011 Elsevier B. V. All rights reserved.