Aqueous M3+/M2+ redox potentials for nine of the ten fourth-period transition metals, M, have been calculated with the use of DFT methodology in combination with the COSMO continuum model. Entropy contributions to the potentials are taken from experiments. The model introduces no adjustable parameters beyond those present in the underlying theoretical models. Inclusion of two solvation spheres (18 water molecules) is necessary. For the ions studied, the average absolute difference from experimental values is 0.29 V, with four out of nine potentials (those of V, Cr, Fe, Cu) reproduced with better than 0.1 V accuracy.