Hydroxyoximes are well established copper extractants that are used in several high-scale in- dustrial installations for producing cathodic copper from oxide ores. The most economically produced copper is obtained by means of hydrometallurgical processes in which hydroxyoximes are used to separate the copper from iron and to concentrate its aqueous solution. The industrial application of hydroxyoxime extractants and their properties have been discussed in several previous papers. *-I 3 Some comprehensive reviews of the chemistry of transition metal complexes with oximes have been presented. 14 v L5 The kinetics and mechanism of metal extraction with hydroxyoximes were also reviewed. 1.11 13,16 The aim of this work is to review the chemistry of metal extraction with hydrophobic hydroxyoxime extractants, with emphasis upon: (1) the characterization of the composition and stability of hydroxyoxime complexes with metals; (2) hydroxyl group acidity; and (3) pH, values, including the effect of hydroxyoxime structure. Appropriate tabular data are presented.