Conventional O-18 isotopic labeling techniques have been used to measure the water exchange rates on the Rh(III) hydrolytic dimer [(H2O)(4)Rh(mu-OH)(2)Rh(H2O)(4)](4+) at I = 1.0 M for 0.08 < [H+] < 0.8 M and temperatures between 308.1 and 323.1 K. Two distinct pathways of water exchange into the bulk solvent were observed (k(fast) and k(slow)) which are proposed to correspond to exchange of coordinated water at positions cis and trans to bridging hydroxide groups. This proposal is supported by O-17 NMR measurements which clearly showed that the two types of water ligands exchange at different rates and that the rates of exchange matched those from the O-18 labeling data. No evidence was found for the exchange of label in the bridging OH groups in either experiment. This contrasts with findings for the Cr(III) dimer. The dependence of both k(fast) and k(slow) on [H+] satisfied the expression k(obs) = (k(o)[H+](tot) +k(oH)K(al))/([H+](tot) + K-al) which allows for the involvement of fully protonated and monodeprotonated Rh(III) dimer. The following rates and activation parameters were determined at 298 K. (i) For fully protonated dimer : k(fast) = 1.26 x 10(-6) s(-1) (Delta H double dagger = 119 +/- 4 kJ mol(-1) and Delta S* = 41 +/- 12 J K-1 mol(-1)) and k(slow) = 4.86 x 10(-7) s(-1) (Delta H double dagger = 64 +/- 9 kJ mol(-1) and Delta S double dagger = -150 +/- 30 J K-1 mol(-1)). (ii) For monodeprotonated dimer : k(fast) = 3.44 x 10(-6) s(-1) (Delta H double dagger = 146 +/- 4 kJ mol(-1) and Delta S double dagger = 140 +/- 11 J K-1 mol(-1)) and k(slow) = 2.68 x 10(-6) s(-1) (Delta H double dagger = 102 +/- 3 kJ mol(-1) and Delta S double dagger = -9 +/- 11 J K-1 mol(-1)). Deprotonation of the Rh(III) dimer was found to labilize the primary coordination sphere of the metal ions and thus increase the rate of water exchange at positions cis and trans to bridging hydroxides but not to the same extent as for the Cr(III) dimer. Activation parameters and mechanisms for ligand substitution processes on the Rh(III) dimer are discussed and compared to those for other trivalent metal ions and in particular the Cr(III) dimer.