Secondary phase inclusions with a size of typically 1-3 mum were observed in polycrystalline sheet silicon samples by preferential etching/Normaski microscopy. The chemical compositions of these inclusions were characterized by secondary ion mass spectroscopy (SIMS) depth profile measurements. It was found that there are mainly two types of inclusions in the sheet Si: nitrogen-rich with trace oxygen, and carbon-rich with trace nitrogen. Fourier transform infrared (FTIR) spectroscopy results indicated that the nitrogen rich particles are either alpha-Si3N4 or silicon oxynitride, while the carbon rich particles are most likely a form of graphite. Microscopic cross-sectional FTIR determined that inclusions deeper in the wafer contain more oxygen than those near the top-surface. Chemical reactions between these included particles and molten Si are discussed based on a thermodynamic analysis. It is concluded that of alpha-Si3N4 particles, formed either through Si-N-2, reaction or incorporated from refractory materials, transform into oxynitrides in the deeper region where the dissolved oxygen exceeds 2.8 x 10(17) cm(-3). (C) 2004 Elsevier B.V. All rights reserved.