Synergistic extraction has been demonstrated for nickel(II) with 5,8-diethyldodecan-6-oxime-7-ol (DEHO), the active component of LIX63 and a range of carboxylic acids including aliphatic and aromatic derivatives. The synergism was shown to depend on the nature of the carboxylic acid with a linear dependance between the pKaof the acid and the synergistic coefficient as defined by the difference in pH50between that obtained by the mixture of extractants and that of DEHO along.