Electrical deactivation of arsenic in highly doped silicon has been studied using the positron‐beam technique. Direct experimental evidence linking the formation of arsenic‐vacancy complexes (i.e., Asn‐v) to the deactivation process is reported. The average number of arsenic atoms per complex, n̄≳2, was determined by comparing the observed complex concentrations with those of the deactivated arsenic inferred from Hall‐effect measurements.