Photoluminescent Er/Si-nanocrystal composites were obtained after annealing Er doped silicon suboxide (SiOx) thin films. The films were prepared by reactive sputtering (Ar/O2 atmosphere) with a pure silicon target partially covered with metallic Er. The presence of Er in the resulting films strongly influences Si nanocrystal nucleation and growth during thermal treatment at temperatures between 300 and 1300 °C. A correlation between Er photoluminescence (PL) spectra, Er speciation and Si nanocrystal properties indicated that PL bands and their intensity are directly influenced by the nanocrystal size and density, and their vicinity to the Er3 + centers. This correlation is explained by considering Er centers as promotor for SiOx disproportionation, locally increasing Si0 concentration which leads to formation of Si nanocrystals in the vicinity of Er.