The incorporation of neptunium(VI) into the layered uranyl selenite Cs[(UO2)(HSeO3)(SeO3)] has yielded the highest level of neptunium uptake in a uranyl compound to date with an average of 12(+/- 3)% substitution of Np-VI for U-VI. Furthermore, this is the first case in nearly 2 decades of dedicated incorporation studies in which the oxidation state of neptunium has been determined spectroscopically in a doped uranyl compound and also the first time in which neptunium incorporation has resulted in a structural transformation.