The distribution coefficient (K-D) of trace nickel on an iminodiacetate-chelating resin Chelex 100 increased with pH to a peak value close to 2 000 000 ml/g at pH of about 5. At higher pH values the K-D dropped to 5000 ml/g due to competition of an unidentified complexing agent, in equilibrium of three-component Na/H/Ni ion-exchange system, H/Ni exchange was found to be dominant in the pH range up to 8 and in Na concentration up to 6 mol/l. The practical ion-exchange capacity for nickel was 2.15 mmol/g (4.3 meq/g). (C) 2000 Elsevier Science B.V. All rights reserved.