The study of optical transitions in lanthanide(III) ions has evolved separately from molecular photophysics, but the framework still applies to these forbidden transitions. In this study, a detailed photophysical characterization of the [Tb(H₂O)₉]³⁺aqua ion was performed. The luminescence quantum yield (φ_lum), excited state lifetime (τ_obs), radiative (k_rA) and nonradiative (k_nr) rate constants, and oscillator strength (f) were determined for Tb(CF₃SO₃)₃in H₂O/D₂O mixtures in order to map the radiative and nonradiative transition probabilities. It was shown that the intense luminescence observed from Tb³⁺compared to other Ln³⁺ions is not due to a higher transition probability of emission but rather due to a lack of quenching, quantified by quenching to O-H oscillators in the aqua ion of k_q(OH) = 2090 s^-1for terbium and k_q(OH) = 8840 s^-1for europium. In addition, the Horrocks method of determining inner-sphere solvent molecules has been revisited, and it was concluded that the Tb³⁺is 9-coordinated in aqueous solution.