The radiative rate constants for depopulation of the excited states of closely-related series of anionic, neutral and cationic europium, terbium and ytterbium complexes have been measured in H₂O and D₂O. With the aid of selective ligand deuteriation, the relative contributions of OH, NH (both amide and amine) and CH oscillators have been measured and critically assessed. Quenching of the Eu ⁵D₀ excited state by amine NH oscillators is more than twice as efficient as OH quenching. The importance of the distance between the excited Ln ion and the XH oscillator is described with recourse to published crystallographic information. The general equation, q = A′(Δk H₂O –k D₂O )_corr is presented and revised values of A′ for Eu (1.2 ms), Tb (5 ms) and Yb (1 µs) given, which allow for the quenching contribution of closely diffusing OH oscillators. The relevance of such studies to the hydration state of certain gadolinium complexes is described and clear evidence provided for a break in hydration at gadolinium.