The effect of arsenate on phosphate determination by the malachite green spectrophotometric method was investigated. The molar absorptivities of the molybdophosphate and malachite green-molybdoarsenate species at 625 nm and a final acidity of 0.38 M were calculated as 10.4 +/- 0.13 x 10(4) and 7.2 +/- 0.17 x 10(4) 1 mol(-1) cm(-1) respectively, indicating that arsenate could interfere in phosphate measurement. Arsenate concentrations as low as 23 mug 1(-1) caused increase in colour development in phosphate solutions. However, the extent of colour development for both anions depended on the final acid concentration of the solution. An acidified sodium sulphite solution (0.83 M NaSO3, 0.83 M H2SO4) quantitatively prevented arsenate colour development up to 300 mug 1(-1) As(V). It was also demonstrated that the method removed As(V) interferences in mixed As/P solutions and therefore can be used to treat natural water samples with elevated arsenate concentrations before phosphate measurement. (C) 2001 Elsevier Science B.V. All rights reserved.