This paper describes the results of the extraction of trivalent lanthanides Ln(III) by supercritical fluid CO2 (SF-CO2) containing N,N,N',N'-tetrabutyl-3-oxapentanediamide (TBODA), one of a group of neutral oxadiamide ligands that have the potential to replace traditional organophosphorus extractants and make the SF-CO2 extraction process more environmentally benign. According to this study, TBODA dissolves in neat SF-CO2 easily, but the solubility of Ln(III)/TBODA complexes in neat SF-CO2 is low and little Ln(III) can be extracted in the SF-CO2 phase. Addition of small amounts of acetone as a modifier in SF-CO2 can significantly improve the extraction of Ln(III) from either aqueous nitric acid solutions or oxide solids. The low solubility of Ln(III)/TBODA complexes in neat SF-CO2 and the improved solubility of Ln(III)/TBODA complexes in acetone-modified SF-CO2 are discussed in terms of the structure of Ln(III)/TBODA complexes and its effect on solute-solvent interactions.