ABSTRACT The extraction of mono- and dicarboxylic acids by octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide [Oπ -D(iB)CMPO] in tetrachloroethylene (TCE) has been studied using 14C labeled acids. It has been found that CMPO primarily behaves as a monodentate ligand, although bidentate behavior may be obtained under certain conditions.