Separation of cobalt(II) and nickel(II) using bis(2-ethylhexyl)phosphinic acid as an extractant has been investigated by a liquid-liquid and a continuous countercurrent extraction. For comparison, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester has also been used, and the results are discussed in terms of cobalt-nickel selectivity. Based on the results, a highly selective procedure using 20% bis(2-ethylhexyl)phosphinic acid in heptane has been proposed to separate zinc(II), copper(II), manganese(II), cadmium(II), magnesium(II) and cobalt(II) from nickel(II). The separation method has been successfully applied to purification of unrefined nickel(II) sulfate solutions.