Abstract: The composition and structure of trialkylphosphine oxide (TRPO) was studied by infrared spectrometry and NMR method. Our experimental results indicate that the extraction is effected by coordination of the lone pair electron of the oxygen atom of P=0, forming an extractable coordination compound. The equilibrium constants for the extraction of water and nitric acid with TRPO and the structure of their complexes are reported.The extraction and back-extraction behavior of neptunium and plutonium in different valence states in the system TRPO-kerosene-HNO3 were also studied. The TRPO is found to be a good extractant for neptunium and plutonium.