The few theoretical treatments which have been successful in predicting the thermodynamic parameters of substitutional impurities are examined for their implications regarding antisite defects, Ba or Ab, and antistructure pairs, BaAb in AB semiconductors. All such theories predict a large concentration of BaAbx, especially in III-Vs. All predict that BA+ or BA+2 and Ab- or Ab-2 should be important to the properties of III-Vs. Results of the Phillips-Van Vechten dielectric two-band model are presented for 23 common semiconductors. These are combined with results for vacancies in the accompanying paper to give a detailed account of native defects in GaP both during crystal growth and as the sample cools. The existence curve is calculated and compared with experiment. The interaction between vacancies and antisite defects is invoked to explain why deviations from stoichiometry have slight effects on carrier concentration but large effects on luminescent efficiency. © 1975, The Electrochemical Society, Inc. All rights reserved.