Titration of an aminophosphonate-chelating ion-exchange resin Duolite C467 in hydronium form with 1 M NaOH in 1 M NaCl reflected the diprotic nature of this exchanger. Two inflection points were detected: at pH values 9.3 and about 6.1. ion-exchange capacity calculated from the second inflection point was 4.80 meg per g of dry hydronium-form resin. Sodium ion uptake exceeded this value by as much as 2.6 meq/g, which is assumed to have been caused by sorption of NaCl electrolyte. Exchange of the less acidic hydronium ion for sodium ion showed a constant cornered selectivity coefficient (K-H) value of -6.5 (logK(H)) in the sodium ion conversion range 50-90%. The corresponding value for the exchange of the more acidic hydronium ion was between -4 and -5 in the sodium ion conversion range 10-50%. Hydrolysis of the sodium-form resin in NaCl solutions does not essentially convert the resin into hydronium form but considerably increases the solution pH.