In the presence of Molybdenum(VI) and Arsenic(III) a new polarographic peak appears at about -1.2 V which corresponds to a hydrogen catalytic wave. It allows trace determination of both ions in the range of 10(-6)-10(-8) M with a detection limit of 2 x 10(-9) M. The optimum working condition was found to be as 0.1 M acetate buffer and pH 3.0. For the determination of one of these ions, the second ion concentration has to be about 500 times higher than the other ion under investigation. There is no serious interference from some cations such as Cr(III), Zn(II), Pb(II), Cd(II), Ni(II), Cu(II), Se(IV) and anions such as bromide, chloride, nitrate, sulfate and sulfite when present 1.0-100 times of arsenic. They did not interfere in the determination of arsenic. In the presence of these ions 10(-8) M arsenic could be determined with great accuracy. The arsenic content in beer has been determined using this method. (c) 2006 Elsevier B.V. All rights reserved.