4-(4-Fluorobenzoy1)-3-methyl-1-phenyl-pyrazol-5-one (HL) has been synthesized and its complexation properties in solution were examined. Mixed ligand chelate extraction of trivalent lanthanoids (La, Nd, Eu, Ho, and Lu) from chloride medium at constant ionic strength mu = 0.1 into C6H6 with HL in combination with one of the three phosphine oxide compounds trioctylphosphine oxide (TOPO), tributylphosphine oxide (TBPO), or triphenylphosphine oxide (TPPO) as well as with tributylphosphate (TBP) was studied. The interaction between the extractants in C6H6-d(6) has been studied by H-1, C-13, and P-31 NMR spectroscopy and its influence on the synergistic extraction of Nd(III) ion has been investigated. The composition of the extracted species was established as LnL(3) center dot HL with HL alone and as LnL(3) center dot 2S in the presence of TOPO, TPPO, and TBP or LnL(3) center dot S with the mixture of HL-TBPO. A synergic effect up to 10(6) was observed for the extraction of the above-mentioned lanthanoid ions with binary mixtures of extractants. The change of the synergistic agent causes a significant increase of the K-L,K-S values. The parameters of the extraction process were determined and the separation factors were calculated.