Metal chelating resins containing aminothiophosphonate ligands were synthesised from two types of starting polymers: vinylbenzyl chloride/divinylbenzene and more polar acrylonitrile/ethyl acrylate/divinylbenzene. Both polymers were first modified with amines (ethylenediamine and diethylenetriamine) and subsequently reacted with diethyl thiophosphonochloridate. The resin based on the more polar polymer, containing 2.61 mmol/g phosphorus and 5.52 mmol/g nitrogen, modified with ethylenediamine was selected as the most promising for Cd/Ni separation. Sorption studies have been performed with 10(-4) M Cd(II) and Ni(II) solutions in 0.2 M acetate buffer in the pH range 3.7-5.6. It has been found that the aminothiophosphonate resin is highly specific for Cd (log K-d = 4-5), whereas Ni sorption is negligible. This new resin is compared with a commercial metal chelating polymer resin, Purolite S950, containing aminomethylphosphonate ligands.