The oxidation kinetics of aquated Cr(II) by H2O2 is accelerated by a factor of 2-5 in the presence of organic and inorganic anions. The reaction was studied in detail as a function of anion concentration, temperature, and pressure for nine different anions. The kinetic data exhibit saturation behavior at high anion concentration, which can be assigned to the formation of a 1:1 complex of the type Cr(II)(H2O)5(An)+. The rate and activation parameters are in line with an inner-sphere electron-transfer mechanism, in which both the inner-sphere complex formation and the electron-transfer rate constants are affected by the presence of the anion in the coordination sphere of Cr(II). Trans-labilization effects and the increased electron density on the metal center are suggested to account for the accelerating effect of the investigated anions.