SiOx films with a nominal s-value (1 less than or equal to x less than or equal to 2) were deposited on flat-surface silicon substrates by reactive r.f, magnetron sputtering at substrate temperatures of 30 and 500 degrees C. respectively. X-ray diffraction and high resolution TEM investigations of SiOx films with x = 1.45 and x = 1 show that as-deposited films have an amorphous structure. After annealing, a nucleation of Si nanocrystals was found with increasing size at increasing initial Si concentration and annealing temperature. The weak photoluminescence in the visible region of as-deposited SiOx films increases remarkably by annealing with dependence on x.