This paper presents the results of the extraction of lanthanum(III) by mixtures of 1-phenyl-3-methyl-4-benzoyl-pyrazalone-5 (HPMBP) and triisobutylphosphine sulfide (TIBPS, B) from a nitrate medium. The mixtures have evident synergistic effects on lanthanum(III). La(III) is extracted as La(NO3)(2)(PMBPB)-P-.-B-. by the mixture instead of La(PMBP)(3)(HPMBP)-H-., which is extracted by HPMBP alone. The equilibrium constants and the thermodynamic functions, i.e., DeltaH, DeltaS, and DeltaG, are determined. The extraction of other rare earth elements, Nd(III) and Dy(III), by mixtures of HPMBP and TIBPS is also studied. The synergistic effect on Nd(III) and Dy(III) is not so high as that on La(III). The possibility of separating La(III), Nd(III), and Dy(III) is discussed according to the separation factors.