Measurements of the proton NMR paramagnetic relaxation rates for several series of isostructural lanthanide (III) complexes have been performed in aqueous solution over the field range 1.0 to 16.5 Tesla. The field dependence has been modeled using Bloch-Redfield-Wangsness theory, allowing values for the electronic relaxation time, T1e and the magnetic susceptibility, μeff, to be estimated. Anomalous relaxation rate profiles were obtained, notably for erbium and thulium complexes of low symmetry 8-coordinate aza-phosphinate complexes. Such behaviour challenges accepted theory and can be interpreted in terms of changes in T1e values that are a function of the transient ligand field induced by solvent collision and vary considerably between Ln3+ ions, along with magnetic susceptibilities that deviate significantly from free-ion values.