EFG silicon ribbon and two kinds of polycrystalline silicon (SILSO and HEM) obtained by casting techniques, with different concentrations of oxygen and carbon impurities have undergone thermal treatments in the temperature range from 450 to 1300°C. Infrared spectroscopy was used to monitor changes in the concentration and configuration of various defects. The carbon-to-oxygen concentration ratio and the thermal history of the investigated materials proved to be of the paramount importance in the nucleation and growth of various precipitation phenomena at higher temperatures. As distinct from single crystal silicon, grain boundaries and high dislocation densities offer an additional source of impurities which may be seen by the infrared techniques upon their activation at higher temperatures.