The equilibrium.extraction behavior for a series of tervalent lanthanide ions (Ln3+) using a chloroform solution containing di(2-ethyl-hexyl) phosphoric acid (HDEHP), diphenylphosphinic acid (HDPP), dibutylphosphorothioic acid (HDBPT), di-n-octylphosphorodithoic acid (HDOPDT), or di(2-ethylhexyl)phosphorodithioic acid (HDEHPDT), either alone or combined with adduct forming agents is studied. The extracted species are Ln(DEHP)3(HDEHP)3, Ln(DPP)3(HDPP)3, Ln(DBPT)3, and are Ln(DBPT)3(HDERP)3in the presence of o-phen and its analogs (B). Extraction constants for the lanthanides follow the order HDPP > HDEHP > HDBPT » HDOPDT. HDPP was the most selective of all the extractants examined. HDOPDT and HDEHPDT were found to be ineffective lanthanide extractants.