Doppler broadening spectroscopy has been performed using a monoenergetic positron beam to investigate the effect on the interface region between nanocrystalline Si and SiO(2) of annealing in nitrogen and oxygen at temperatures between 400 and 900 degreesC. Both nitrogen and oxygen are trapped in voids at the interface at the lower annealing temperatures. Above 700 degreesC nitrogen and oxygen react with the nanocrystals; the resulting volume increase introduces stress in the SiO(2) matrix which is relaxed by the shrinkage of its intrinsic open volume. With supporting evidence from photoluminescence spectroscopy it appears that agglomeration of Si nanocrystals occurs more readily during annealing in oxygen than in nitrogen.