Abstract A sensitive and selective voltammetric method for simultaneous determination of Sb(III) and Mo(VI) using Quercetin (Q) as complexing agent is described. Optimal conditions were found to be: pH 3.7, C Q =6.0 µmol L −1 and E acc =−0.10 V. The LOD (3 σ ) for Sb(III) are 0.076 and 0.040 µg L −1 , whereas for Mo(VI) are 0.086 and 0.048 µg L −1 with t acc of 60 and 120 s, respectively. The method was validated using synthetic sea water (ASTM D665) and was applied to the determination of Sb(III) and Mo(VI) in natural waters with satisfactory results.