A new and simple differential pulse polarographic method was developed for the trace determination of As(III) and As(V) species in the same sample. Among several electrolytes, 0.1 M HCl was found to be the most suitable. As(III) had 3 peaks in this medium, and the peak at -0.82 V was chosen because of its peak shape and response to standard additions. In this medium, the limit of detection for As(III) was found to be 1.5 x 10(-7) M. Since As(V) is not electroactive, it had to be reduced first into As(III). For this purpose, KI in an acidic and hot medium was found to be suitable. For As(V), the limit of detection was 1.0 x 10(-7) M. The interference effects of the most common cations, such as Fe(III), Cu(II), Pb(II), and Zn(II), and of anions such as sulfate, chloride, and nitrate ions, were investigated. None of these ions had an overlapping peak with the peaks of As(III) in this medium. However, it was found that only Fe(III) could oxidize As(III). This creates problems in As(V) determination, since As(III), which was formed from the reduction of As(V), will be oxidized by Fe(III) and As(V) determination will not be correct. This problem was eliminated with KI, added for the reduction of As(V). As(III) and As(V) concentrations for a sample containing 1 x 10(-5) M As(III) and 1 x 10(-5) M As(V) could be determined with high accuracy. This method was applied to the tap water of Ankara.