Equilibria and thermodynamic data for cerium dimer species were calculated for both the Ce-H2O-O-2 and Ce-H2O-H2O2 systems. It was found that Ce (IV) has a tendency to form dimers, especially when the total cerium concentration is higher than 0.001 M. There are three major soluble dimer forms considered in this paper: (CeOCe)(6+), (CeOCe)(OH)(5+), and (CeOCe)(OH)(2)(4+), all having very similar standard potentials (E(Ce(IV)dimer/Ce(III))(0.)1.66 for all 0.02 V.) When they coexist in solution, the concentration ratio of the Ce (IV) dimer to Ce3+ depends on the total cerium concentrations, the oxidizing agent, and in some cases the pH. Increasing the value of each individual factor favors the formation of dimers. It was also found that the direct oxidation of Ce3+ to dimers (CeOCe)(OH)(x)((6-x)+) by O-2 appeared to be insignificant, but the oxidation of Ce3+ to the three dimers by H2O2 would be much more favorable. (c) 2005 The Electrochemical Society.