In this work previous static and flow injection voltammetric methods for the determination of phosphate and silicate have been developed for the determination of total phosphate and soluble silicate in commercial washing powders. The method is based on the electrochemical reduction of heteropoly acids at + 0.3 v. (vs. SCE) at a glassy carbon electrode. Anionic detergents are a constituent of washing powders and are found to reduce markedly the differential pulse voltammetric peak currents obtained for 12-molybdophosphate and 12-molybdosilicate in aqueous solutions. Use of a sample solution 40–50% in ethanol was found to prevent this interference. By this method soluble silicate has been determined in unhydrolysed solutions of washing powders and total phosphate in hydrolysed solutions using both static and flow injection modes.