Cu-Y binary oxysulfide was synthesized for the first time. A series of batch experiments were conducted to determine the optimum conditions for preparing the adsorbent and further investigate the effects of the initial pH and coexisting anions on the removal of arsenic. The results indicate that the adsorbent displays an extremely excellent performance in removing both As(III) and As(V). The maximal adsorption capacities at the initial pH 6.0 are 263.16 and 270.27 mg/g for As(III) and As(V), respectively. The adsorbent remains relatively stable even at the initial pH 3 and at this pH it also exhibits a very high removal efficiency for As(V). Based on Fourier transform infrared (FTIR) spectrometer and X-ray photoelectron spectrometer (XPS) analyses and other results obtained in our experiments, a possible mechanism for the removal of arsenic is proposed. The dissolved O-2 is activated to O-2(center dot-) via a single-electron transfer reaction from Cu(I) on the adsorbent, which oxidizes As(III) to As (V). Then, arsenate anions are adsorbed onto the surface of the adsorbent and the adsorbed arsenate anions further react with the hydroxyl groups on the surface of the adsorbent via an ion exchange or precipitate with Y (III) on the surface of the adsorbent.