The distribution constant KD(HTTA) of thenoyltrifluoroacetone between 10–3M HNO3 and cyclohexane was determined by means of spectrophotometric measurements of HTTA concentration in the aqueous phase. The distribution ratio, D, of HTTA, when tri-n-octylphosphine (TOPO) is present, and the equilibrium constant,n
, of the reaction between HTTA and TOPO in the organic phase were also determined. By means of the known KD(HTTA) and D values, the equilibrium constant of the HTTA-TOPO interaction was calculated. Making use of KD(HTTA) andn
values and of the slope analysis method, the composition of the extracted lanthanide complexes was established. By considering the interaction reaction between the extractants, the species Ln(TTA)3 · TOPO and Ln(TTA)3 · 2(TOPO), for Ln=La and Yb, were identified in the organic phase. The equilibrium constants of the reactions that give rise to the species were also calculated.