Abstract The deviation from stoichiometry or nonstoichiometry characterises a homogeneity range of chemical compounds. It is determined as the difference in the nonmetal- to metal atoms ratio between a real A n B m+ δ ( δ><0) and stoichiometric A n B m composition. Nonstoichiometry creates defects which have an effect on all properties of a crystal. The classification of defect is given. The enthalpies and entropies of quasi-chemical reactions describing the defects composition of III–V compounds are presented and give the possibility to estimate the concentration of the major defects for the solubility limits of III–V compounds.