The solvent extraction of nickel from acidic solutions by pyridinecarboxylate esters (2-, 3- and 4-C5H4N.CO.OR) mixed with organophosphorus acids (R(2)POOH, (RO)RPOOH and (RO)(2)POOH) in toluene was investigated for both series of compounds in which R = n-octyl, 2-ethylhexyl and cyclooctyl. Substantial synergistic effects were observed, which increased in the orders : pyridine 2-ester < 3-ester < 4-ester, and : phosphinic < phosphonic < phosphoric acid. The extractability of divalent base metals from sulphate solutions by mixtures of isodecyl 4-pyridinecarboxylate and di(2-ethylhexyl) phosphoric acid in Shellsol K decreases through the series Cu > Ni > Zn, Co > Ca > Mg. In a batch countercurrent experiment, a simulated leach liquor containing Ni 2.1, Cu 0.5, Ca 0.4 and Mg 5.0 g dm(-3) (initial pH 3.00) was extracted with the mixed reagent (0.50 M) in four stages at unit phase ratio, without pH adjustment. Recoveries of nickel and copper were 93 and 100%, with co-extractions of calcium and magnesium of 10 and 1%, respectively. In a similar experiment using isodecyl 3-pyridinecarboxylate in place of the 4-isomer, the overall extractions were nickel 80, copper 100, calcium 17 and magnesium 3%.