Phillips and Van Vechten have shown that, because of the high valence electron density in diamond type semiconductors, their vacancies may properly be treated as macroscopic cavities. With this model one may estimate the enthalpy of neutral vacancy formation, AH(VX), as simply the surface energy of the cavity. Values of AH(VX) are known for Si and Ge and are 10% less than the theoretical estimates. Here this model is extended to zinc-blende and wurtzite type crystals. The virtual-enthalpies of formation AH(Vax) and aH(Vbx) are calculated for 17 common semiconductors. Difficulties in analyzing empirical data which arise from the ionization of the vacancies and their interaction with other defects are noted. However, for CdS, where the data are adequate, the empirical values are about 12% less than those calculated. © 1975, The Electrochemical Society, Inc. All rights reserved.