Lanthanide luminescence has been treated separate from molecular photophysics, although the underlying phenomena are the same. As the optical transitions observed in the trivalent lanthanide ions are forbidden, they do belong to the group that molecular photophysics has yet to conquer, yet the experimental descriptors remain valid. Herein, the luminescence quantum yields (ϕlum), luminescence lifetimes (τobs), oscillator strengths (f), and the rates of nonradiative (knr) and radiative (kr ≡ A) deactivation of [Eu(H2O)9]3+ were determined. Further, it was shown that instead of a full photophysical characterization, it is possible to relate changes in transition probabilities to the relative parameter Arel, which does not require reference data. While Arel does not afford comparisons between experiments, it resolves emission intensity changes due to emitter properties from intensity changes due to environmental effects and differences in the number of photons absorbed. When working with fluorescence this may seem trivial; when working with lanthanide luminescence it is not.