The activity coefficients of four extractants belonging to the carbamoylmethylphosphoryl class were measured by vapor-phase osmometry. All of the activity coefficients were within 15% of unity for concentrations up to 0.5Min toluene. As the basicity of the extractant's phosphoryl-group increases, its activity coefficient decreases. Octylphenyl-N,N-diisobutylcarbamoylmethyl-phosphine oxide [OφD(iB)CMPO] in toluene exhibited a marked decrease in activity coefficient upon equilibration with water or nitric acid. This helps explain a negative deviation from a third-power extractant dependency exhibited by americium distribution ratios, DAM, from 0.500 MHNO3. A positive deviation from a third-power extractant dependency, of DAMfor Am extracted from 5.00MHNO3, is explained by the alteration of the diluent's physico-chemical properties as its nitric acid content Increases. The extraction of nitric acid and water by toluene solutions of 0φD(iB)CMPO was also measured.