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., As-n-v) to the deactivation process is reported. The average number of arsenic atoms per complex, (n) over bar>2, was determined by comparing the observed complex concentrations with those of the deactivated arsenic inferred from Hall-effect measurements. (C) 1995 American Institute of Physics.