A separation process for rare earth elements (Pr/Nd and Y/Er) by liquid-liquid extraction with bis(2-ethylhexyl) phosphinic acid is investigated. Extraction equilibrium formulations for these elements are established and extraction constants are determined employing kerosene and toluene as diluents, in which <(MR3(RH)(3))over bar> is formed at low loading, and then the larger aggregated species, <((MR3)(RH)(4))over bar>, appears with increasing loading ratio. The proposed formulations and constants successfully express the extraction behaviors over the whole range of loading ratio up to 0.4 in single element systems. These are valid for binary element systems of Pr/Nd and Y/Er and also for the distribution behaviors in scrubbing treatment. The separation is enhanced at high pH region, that is, at high loadings. Scrubbing is also effective for separation. Simulation of Pr/Nd separation by continuous counter-current mixer-settler cascades shows that separation is enhanced by employing appropriate pH control and scrubbing treatment.