The fluorescent properties of the tris chelates of europium and various β-diketones have been examined. Fluorinated ligands give better sensitivity than unfluorinated ligands, but the best sensitivity is found with 2-thenoyltrifluoroacetone in dimethylformamide. A rapid method for the determination of 10–7 to 10–8 M europium(III) is discussed. Both europium and terbium can be readily determined in the 10–2 to 10–3 M range by measurement of the fluorescence of the complexes formed with dimethylformamide alone.