Increasing the steric hindrance of the substituents on the amidic nitrogen of the carbamoylmethylphosphine oxide class of extractants leads to a significant decrease in the tendency of these compounds to form a third phase during the extraction of nitric acid. An increase in steric hindrance leads to a reduction in the accessibility of the nitrogen and in the dipole-dipole and dipole-induced-dipole attractions between extractant molecules, thereby reducing the tendency to form a third phase. A corresponding increase in the amount of uranium that could be extracted without third phase formation was not observed due to a reduction in the interaction of the extractant with the diluent because of coordination with the metal ion by both polar groups.