A systematic study on the synthesis of silicon nanoclusters formed in plasma-enhanced chemical vapor deposited substoichiometric silicon oxide films by annealing at 1100 °C for 1/2 h as a function of different deposition parameters, has been performed. The clustered Si volume fraction is deduced from a fit to the experimental electron energy loss spectrum. At any deposition condition, the clustered silicon concentration is significantly lower than the initial silicon excess concentration, demonstrating that high temperature anneal of SiOx films induces only partially the phase separation between Si and SiO2. This effect, explained by taking into account the strain energy associated with the different atomic densities of Si and SiO2, has been confirmed through a detailed study of the host material by scanning a 2 Å electron beam among agglomerated silicon dots, showing that the matrix it is not a stoichiometric SiO2 but …