Abstract When selenite was added into solutions of some ions such as copper, lead, cadmium, zinc, chromium and arsenic their DPP peak decreased. This kind of interference was observed in all acids and in wide range of pH values, causing at least 50% error when both ions (the ion present and selenite) were in similar quantities. It was found that this interference was not taking place at pH 8.5 in the presence of large amounts of selenite. A synthetic sample containing these elements in 10 −5 M in the presence of 50 times selenite has been analyzed at pH 8.5 without any interference. The detection limit in this medium was 3 × 10 −7 M for cadmium and lead, 1 × 10 −6 M for copper and zinc and it was 3 × 10 −7 M for selenite.