The precipitation behavior of copper in silicon single crystals containing different amounts of oxygen has been investigated within the temperature range of 900°–1300°C. Precipitation which occurs during the cooling period on structural defects and at random sites throughout the bulk crystal has been detected by infrared transmission microscopy. The shape and crystallographic relationship of the precipitate as related to the silicon matrix was determined. The reliability of the method of copper decoration for the determination of low dislocation densities in the presence of oxygen was investigated by comparing copper decoration pictures with x‐ray diffraction photomicrographs.