Stopped-flow studies of the calcium-arsenazo III (Ar) system exhibit reaction time constants ranging from 4 to 20 ms, mainly caused by the formation of CaAr and CaAr 2 complexes ( [ Ca] 0 = 10 μ M to 10 mM, [ Ar] 0 = 10 μ M to 50 μ M ). At total concentrations [ Ca] 0 ≥ 100 μ M an additional slow reaction mode with time constants of 100 to 600 ms has been found, which is attributed to the formation of the Ca 2Ar complex according to Ca + CaAr ⇌ harp over l Ca 2Ar. The rate coefficients of association and dissociation are, respectively, k + = 900 M −1 · s −1 and k − = 1.8 s −1 , yielding K = k +/k − = 500 M −1 . In biophysics and cell biology arsenazo III may be used to monitor the kinetics of micromolar Ca-concentration changes in the time range ≥ 50 ms . If the optical signal is properly deconvoluted, the time range may be extended to 5 ms. An improved method for purifying arsenazo III is communicated.