The defect formation in the compounds of AIIIBV and AIVBV is considered in the frames of the quasichemical reaction method. When choosing the model of defect formation it has been taken into account the experimental dependences of electrophysical properties on the conditions of their obtaining and thermal dissociation data together with known literature data. Analysis of our results indicates that AIIIBV compounds prove to be double-sided phases with a distinct asymmetry of homogeneity range. InAs has a maximum width of homogeneity range, while for InSb the deviation from stoichiometry is minimal due to the formation of the antistructural defects In sb Predominance of antistructural defects is characteristic for semiconductors of AIVBV group. GeAs and GeP are single-sided unsaturated phases with an excess of germanium and the stoichiometric composition is never realised in these compounds independent on conditions of their production.