Scanning transmission electron microscopic (STEM) analysis has been applied to the study of equilibrium segregation of As to grain boundaries in Si. Direct quantitative evidence for the extant of segregation has been obtained in a number of specimens heated to temperatures between 1000 and 700°C. A value for the binding energy of an As atom to an Si grain boundary of 0·66 eV has been calculated. The experimentally observed saturation concentrations of As have also been shown to be consistent with the density of potential segregation sites in previously proposed models of grain boundary structure in Si. © 1985 Taylor & Francis Group, LLC.