The synergistic solvent extraction of nickel and cobalt by pyridinecarboxylate esters (2-, 3- or 4-C5H4N.CO.OR) in admixture with 3-bromo-or 3-nitro-derivatives of 5-alkylsalicylic acids was studied. Nickel is extracted more strongly than cobalt in all cases and, for systems containing a given pyridinecarboxylate, the pH,, values (pH values for 50% extraction) of both metals decrease in the order : alkylsalicylic acid > bromo-derivative > nitro-derivative. For systems containing a given salicylic acid, the separation between the pH(50) values for nickel and cobalt was found to increase in the order : pyridine 2-ester < 4-ester approximate to 3-ester. The extractability of divalent base metals from sulphate solutions by mixtures of isodecyl 3- or 4-pyridinecarboxylate and 3-bromo- or 3-nitro-5-nonylsalicylic acid in Shellsol K decreases through the series : Cu > Ni > Co approximate to Zn > Ca > Mg. In single-stage batch extraction experiments with a simulated leach liquor containing Ni 2.2, Cu 0.5, Ca 0.4 and Mg 5.0 g dm(-3) (and smaller amounts of other base metals), adjustment of the equilibrium pH value to between 3.3 and 4.0 with magnesium oxide gave extractions of nickel and copper of 97-100%, with co-extractions of calcium and magnesium of < 0.5 and < 0.1%, respectively. Amongst the metals present in lower concentrations, manganese (2-5%) and lead (5-10%) were extracted only slightly whereas cobalt (40-80%), zinc (15-65%) and iron (100%) were more strongly extracted.