An iron(III)-loaded chelating resin (Fe-LDA) with lysine-N-alpha,N-alpha-diacetic acid functional groups has been prepared and its adsorption characteristics for arsenic(III) and arsenic(V) have been examined. Arsenic(V) was strongly adsorbed to the resin in the pH ranee from 2 to 4, while arsenic(III) was moderately adsorbed between pH 8 and 10. The isotherm data for arsenic(V) at pH 3.5 fitted well to a Langmuir equation with a very large binding constant of 7.2 x 10(4) dm(3) mol(-1) and a capacity constant of 0.74 mmol g(-1). The data for arsenic(III) at pH 9 also fitted to a Langmuir equation, with a binding constant of 190 dm(3) mol(-1) and a capacity constant of 0.84 mmol g-(1). Regeneration of the resin was successfully carried out with 0.1 mol dm(-3) sodium hydroxide solution. Both arsenic compounds can be almost quantitatively recovered from the resin under these conditions. Only a small amount of ferric ions (less than 0.1%) was observed to come off the resin during the regeneration with alkaline solutions. Since the Fe-LDA resin showed little affinity for arsenic(III) in acidic media, the present adsorption system can provide satisfactory separation of arsenic(V) from arsenic(III). Arsenic(V) was successfully concentrated in the column packed with Fe-LDA resin from its dilute solution.