A new neptunium, plutonium, and uranium separation process using n- and iso-butyraldehydes as reductantsfor Np(VI) and Pu(IV), respectively, is described for nuclear fuel reprocessing. A kinetics study and a chemical flow sheet study are conducted to develop the selective separation process for neptunium, plutonium, and uranium. In the kinetics study, it is found that n-butyraldehyde reduces Np(VI) to Np(V) in the Purex solution but does not reduce Pu(IV) and U(VI), and iso-butyraldehyde reduces Np(VI) and Pu(IV) but does not reduce U(VI). Based on these results, a new process to separate neptunium, plutonium, and uranium selectively is proposed. The process consists mainly of three steps: the codecontamination step, the neptunium separation step [in which Np(VI) extracted by a solvent of 30% tri-n-butyl phosphate (TBP)/n-dodecane together with Pu(IV) and U(VI) is reduced to Np(V) by n-butyraldehyde and is back-extracted from the solvent], and the uranium/plutonium (U/Pu) partition step using iso-butyraldehyde as a Pu(IV) reductant. In the chemical flow sheet study, the effectiveness of the separation process is demonstrated by the use of miniature mixer-settlers. In the neptunium separation step, ∼99.98% of the neptunium extracted by the 30% TBP/n-dodecane solvent along with U(VI) in the uranium/neptunium coextraction step is reduced by n-butyraldehyde and separated from the uranium stream. In the U/Pu partition step, >99% of the plutonium is reduced by iso-butyraldehyde and separated from the uranium stream.