Extn. sepn. of Ni(II) from Cd(II) ions in sulfate solns. was run with di(2,4,4-trimethylpentyl)phosphinic acid as extractant. Extn. equil., lab. simulation of a 3-stage countercurrent mixer-settler (M-S) extn.-stripping process, and transportation of the metal ions through a polymer inclusion membrane (PIM) were examd. The ions were selectively sepd. by the MS or the PIM technique within a pH 5.5-6.0 with the extractant used in 200-300% excess over stoichiometry.