A study is reported of the extraction of cobalt(II) from oxalate solutions by Aliquat 336 in chloroform. Equilibrium studies by two-phase titration show that the predominant organic-phase complex has the formula [(R3MeN)2Co(C2O4)2]. It is proposed that this species is formed by extraction of the anionic complex Co(C2O4)22- involving interfacial reactions. There is evidence for the formation of a small amount of the complex [(R3MeN)4Co(C2O4)3]. The extraction mechanism proposed is based on a study with the single-drop technique, and is supported by interfacial tension measurements and measurements with a stirred cell of constant interfacial area.