The temperature dependence of the ⁵D₄ Tb ³⁺ level in crystalline Tb(dpm)3 has been determined and its anomalous behaviour has been interpreted in terms of thermal depopulation to a low lying triplet level in the solid complex, Comparisons are made with the behaviour shown by the adducts of this compound in the solid state and in solution. In solution molecular weight and conductance measurements indicate some dissociation of the monomeric units. Large variations in the quantum efficiency of Tb³⁺ tetrakis compounds differing only in the nature of the cation, have been observed. Thermal depopulation of the ⁵D₄ Tb³⁺ level to the ligand triplet level has been established in two series of terbium β-diketoenolates (hfaa and tfaa). This is insufficient to completely explain the quantum efficiency differences. Relative quantum yields of these compounds and related europium compounds in solution and in the solid state have been measured and these results are discussed. Spectroscopic and analytical evidence of 9-coordination in adducts of tetrakis β-diketoenolate compounds is presented and the isolation of some such compounds is reported. The ability of tetrakis compounds to increase their coordination sphere to nine in solution is demonstrated using an nmr technique. The occurrence of intermolecular energy transfer in solution containing lanthanide chelates has been observed. The possibility of triplet- triplet transfer has been investigated and the data indicate that this is possible over relatively short distances. Lanthanide- lanthanide transfer has been established and the results Eire consistent with a diffusion controlled process. The efficiency of this process is relatively low. Intermolecular energy transfer in some crystalline chelates has been observed and is interpreted in terms of triplet-triplet and lanthanide-lanthanide transfer processes.