A hydrophilic thiol resin, poly(ethylene mercaptoacetimide) (PEM), has been prepared from branched polyethyleneimine of molecular weight 40,000-60,000 by Schotten-Baumann reaction using mercaptoacetyl chloride. The resin with a free mercaptan content of 8.26 meq/g and a standard potential of 0.217 V, as determined by electrochemical measurements, exhibits spontaneous redox sorption of arsenate anions in acidic medium. The saturation capacity for arsenate is 106 mg As/g dry resin at pH 2. The resin. however, sorbs arsenite anions only in alkaline medium and shows a saturation capacity of 30 mg As/g dry resin at pH 8. In addition to redox sorptions, a significant amount of arsenic sorption appears to take place via alternative mechanisms such as complexation by thiol and anion exchange on protonated amine sites of the branched PEM. The sorption of both arsenate and arsenite is significantly reduced by the presence of salts, NaCl and Na2SO4, in solution. The sorption kinetics is controlled by the diffusion of arsenical anions in the resin particle. The sorbed arsenic species are readily stripped by 0.2 N NH4OH and the stripped resin in the oxidized (disulfide) form is reconverted to the active thiol form by treatment with an excess of 10% sodium bisulfite solution