Positron lifetime investigations of vacancy distributions in ingots of silicon have shown that vacancies are retained after growth at nearly constant concentrations close to 3×1016cm−3. The vacancies are generally monovacancies and are suggested to be trapped by oxygen clusters. Trapped divacancies can also be formed but they are unstable upon heat treatment at 1000 °C for 16 h. This observation is invoked to explain anomalous oxygen precipitation. This heat treatment has little effect on the distributions of monovacancies in the ingots investigated, so the complexes between vacancies and oxygen clusters are suggested to be formed at temperatures above 1000 °C during the growth. ©1995 American Institute of Physics.