The proposed method for the determination of phosphorus is based on the extraction of molybdophosphoric acid. A mixture of methyl-iso-butyl-ketone and cyclohexane is used as extractant. The addition of cyclohexane allows the separation from silicon. Sodium chlorate is used as oxidant and gives rise to a catalytic current. The voltammetric behaviour of molybdophosphoric acid is described, comparing the dropping mercury electrode and the glassy carbon electrode. The 3σ-detection limit is 2.3 · 10−8 mol P/l. The method was applied to water samples from the river Spree.