Water exchange from the oxo-centered rhodium(III) trimer, [Rh-3(mu(3)-O)(mu-O2CCH3)(6)(OH2)(3)](+), was investigated using variable-temperature (272.8-281.6 K) and variable-pressure (0.1-200 MPa) O-17 NMR spectroscopy. The exchange reaction was also monitored at three different acidities (pH = 1.8, 2.9, and 5.7) in which the molecule is in the fully protonated form (pK(a) = 8.3 (+/- 0.2), l = 0.1 M, T = 298 K). The temperature dependence of the pseudo-first-order rate coefficient for water exchange yields the following kinetic parameters: k(ex)(298) = 5 x 10(-3) s(-1), Delta H-double dagger = 99 (+/- 3) kJ mol(-1), and Delta S-double dagger = 43 (+/- 10) JK(-1)mol(-1). The enhanced reactivity of the terminal waters, some 6 orders of magnitude faster than water exchange from Rh(H2O)(6)(3+), is likely due to trans-labilization from the central oxide ion. Also, another contributing factor is the low average charge on the metal ions (+0.33/Rh). Variation of reaction rate with pressure results in a Delta V-double dagger = +5.3 (+/- 0.4) cm(3) mol(-1), indicative of an interchange-dissociative (I-d) pathway. These results are consistent with those published by Sasaki et al. who proposed that water substitution from rhodium(III) and ruthenium(III) oxo-centered trimers follows a dissociative mechanism based on highly positive activation parameters.