The structures of the (η5-pentamethylcyclopentadienyl)rhodium aqua complexes, as a function of pH, were studied by 1H, 13C, 17O, and 2D NOESY NMR spectroscopic techniques as well as by FAB mass spectrometry and potentiometric titration. The starting complex for our NMR experiments, [Cp*Rh(H2O)3](OTf)2, 1, was structurally characterized by single-crystal X-ray crystallography [130 K, Mo Kα radiation, λ = 0.710 73 Å, a = 23.979(9) Å, b = 9.726(4) Å, c = 18.257(6) Å, Z = 8, orthorhombic, space group Pna21, 3879 independent reflections, R = 0.0482, Rw = 0.1062]. Both 1H and 13C NMR titration experiments of the starting complex, 1, were performed by dissolving 1 in H2O (D2O) and obtaining spectra from pH 2-14. From pH 2–5 only one Cp* signal (1H NMR, 1.57 ppm; 13C NMR, 5.78 ppm) was observed, which was attributed to 1. As the pH of the solution with 1 was increased from 5 to 7, a dynamic and rapid equilibrium was observed to provide putative [Cp*Rh(μ-OH)(H2O)]2(OTf)2, 2, and [(Cp*Rh)2(μ-OH)3](OTf/OH), 3; unfortunately, only one 1H or 13C NMR signal for Cp*Rh at 1.50 (Cp*) or 5.41 ppm (C-CH3), respectively, was found for the latter two species, with broadening of the signals at pH 5.5-6, indicating that conversion from putative 2 to 3 was very fast on the NMR time scale. As the pH was further increased from 7 to 10, only the 1H or 13C NMR signal for 3 was observed at 1.50 or 5.41 ppm, respectively. In addition, starting the equilibrium from 3 (3 ⇌ 1 via putative 2) within the pH range 14–2 provided similar results. The 2D NOESY NMR exchange phasing experiments at pH 5.8 and 11 showed correlations between the Cp* CH3 groups and the H2O or μ-OH groups attached to Rh and between both Cp* CH3 groups of the Cp*Rh aqua complexes, although separate signals for bulk H2O and μ-OH or H2O ligands bonded to Rh were not observed due to a rapid exchange process. A potentiometric titration study gave further evidence that the conversion of 1 ⇒ 3 via putative 2 occurs rapidly with only one pKaof 5.3 being observed, reaffirming the fact that the conversion of 1 ⇒ 3 via putative 2 was extremely fast. The pseudo-first-order rate of conversion of 1 ⇒ 3 at pH 5.8 was measured by an NMR spin population transfer technique to be k1 = 7.18 s-1 (1, 0.034 M; T1 = 1.6 s), while k-1, 3 ⇒ 1, was found to be 2.93 s-1 (T1= 1.5 s). The equilibrium constant, Keq, at pH 5.8 for 1 ⇒ 3 was found to be 353. 17O NMR studies again showed that H2O molecules bonded to Cp*Rh and those in the bulk solution are in very fast exchange (k > 8150 s-1).