The extraction of lanthanides from aqueous solutions using CO2 modified with tributyl phosphate (TBP) and TBP/di(2-ethylhexyl) phosphoric acid (D2EHPA) has been investigated. The aqueous phase used in this study comprised 0.05 M lanthanides nitrate, 0.1 M HNO3, and 3 M NaNO3. Extraction isotherms for La and Gd at 40 degrees C and 150 bar have been determined at various modifier concentrations in CO2. The effect of diluent, modifier concentration, temperature, and pressure on the extraction and separation of the lanthanides (Lns) was also studied. When extracting with TBP the extraction of light Lns initially increases with increasing atomic number and then decreases. Upon increasing the concentration of TBP the amount of Ln extracted also increased. The extraction and separation of Lns increased dramatically with the addition of D2EHPA to the CO2/TBP mixture. The level of extraction obtained with CO2/TBP/D2EHPA at 40 degrees C and 150 bar generally increased with atomic number, with values ranging from 23% for La to 94% for Yb. The separation factors between Lns was generally greater in TBP/D2EHPA. The results of this study illustrate the considerable potential of employing dense gas CO2 modified by organophosphorous complexing agents for the processing of Ln metals.