Abstract The complex formation between molybdate and Alizarin Red S (ARS) has been studied by spectrophotometry and pH titration. Results by the former method confirm that the composition of the complexes formed corresponds to a ratio of 1:1 for molybdate to ARS and that the complexes are stable between pH 2 to 6. Those by the latter are summarized by the following stability constants at 25 °C and an ionic strength of 0.1: MoO42−+ars3−+2H+=MoO3ars3−+ H2O, log(β1⁄mol−3L3)=20.18±0.01; MoO42−+ars3−+3H+=MoO3(Hars)2−+H2O, log(β2⁄mol−4L4)=24.93±0.04 (the completely deprotonated species of ARS is denoted by ars3−, and the concentration of H2O is not included in the definition of stability constants). Protonation constants of MoO42− and ars3− were also determined under the same conditions.