[en] Though extraction equilibria of metals in systems containing monocarboxylic acid have been studied until now in great detail, the influence of the nature of diluents were less extensively investigated. Results are given on the extraction of Fe(III) with n-caprylic acid dissolved in n-decane, benzene, carbon tetrachloride, 1,2,4-trichlorobenzene, i-amyl alcohol, i-amyl acetate, diisopropyl ketone and in a mixture of nitrobenzene and carbon tetrachloride (5:1) as well as results on the extraction of Ni(II) using n-decane, benzene, carbon tetrachloride and 1,2,4-trichlorobenzene as diluents. In all cases information was obtained on the composition of the extracted compounds. The distribution ratios were determined by radiochemical methods. The investigations of the extractant distribution between an aqueous sodium sulphate solution and n-decane, benzene, i-amyl alcohol, i-amyl acetate and diisopropyl ketone as diluents have been performed using ¹⁴C-labelled n-caprylic acid. As a result of these investigations pieces of information were obtained on the state of the extractant in the organic phases. (T.G.)