The nucleation process of oxygen precipitates in silicon at 650 degreesC was investigated using photoluminescence (PL) measurements, which are demonstrated to provide a highly sensitive tool for the characterization of precipitates differing in structure and shape. The results of this study were first analyzed from a thermodynamic and kinetic point of view in order to explain the effect of minute differences in oxygen concentration on the PL spectra, which indicates that two kinds of nanometric oxides coexist in dynamic equilibrium. Then a hypothesis is advanced about the origin of the oxygen-induced PL bands, which is explained in terms of carrier confinement effects induced by the strain at oxide precipitates. (C) 2004 The Electrochemical Society.