The determination of arsenic(III) and arsenic(V), as inorganic arsenite and arsenate, was, investigated by CE with capacitively coupled contactless conductivity detection (CE-C-4 D). It was found necessary to determine the two inorganic arsenic species separately employing two different electrolyte systems. Electrolyte solutions consisting of 50 mM CAPS/2 MM L-arginine (Arg) (pH 9.0) and of 45 mM acetic acid (pH 3.2) were used for arsenic(III) and arsenic(V) determinations, respectively. Detection limits of 0.29 and 0.15 mu M were achieved for As(III) and As(V), respectively by using large-volume injection to maximize the sensitivity. The analysis of contaminated well water samples from Vietnam is demonstrated.