Lanthanon chelates of 3-methyl-1-phenyl-4-trifluoroacetylpyrazol-5-one (HPMTFP) have been synthesized and investigated. Characterization was by elemental analysis, conductivity measurements, Karl Fischer titrations, i.r. and proton magnetic resonance spectra. The complexes are shown to be neutral chelates with the empirical formula Ln(PMTFP) 3 · xH 2O · yC 2H 5OH where for Ln = La, Nd, Eu, Dy, Er, Pr and Lu, x = 2 and y = 0 while for Ln = Y and Gd, x = 2 and y = 1 2 . 1H, 13C and 19F NMR, i.r. and u.v.—visible spectral data together with assignments are presented. Sinha's parameters have been calculated and discussed for Nd and Er.