In the presence of selenium(IV) and molybdenum(VI) a new polarographic peak appears which corresponds to a hydrogen catalytic wave. By differential pulse polarography a single, sharp peak at about -1.1 V is obtained, allowing trace determination of selenium(IV) and molybdenum(VI) in the range 1 x 10(-6)-5.0 x 10(-9) M with a linear calibration and a detection limit of 1.5 x 10(-9) M. The optimum conditions are found to be 0.1 M KNO3 and a pH of about 3.2 (Britton-Robinson buffer). There is no serious interference from some ions when present at 1.0-40 times that of molybdenum. At higher amounts of interfering ions the interference is eliminated by the addition of EDTA. (C) 1999 Elsevier Science B.V. All rights reserved.