Laboratory experiments showed that neptunium hexafluoride is sorbed more effectively by sodium fluoride at 200°C than by the fluorides of lithium, magnesium, and calcium at 100 to 400°C. The equilibrium pressure of NpF6 over the complex formed with sodium fluoride in the presence of fluorine was measured. A sorption-desorption method based upon the difference in equilibrium pressures of the hexafluorides of neptunium and uranium over the sodium fluoride complex does not appear to be useful for separating neptunium hexafluoride from uranium hexafluoride at neptunium: uranium weight ratios that usually exist in spent nuclear fuels. However, favorable results were obtained with a method that involves cosorbing the neptunium and uranium hexafluorides, reducing the neptunium in the NpF6-NaF complex, desorbing the uranium, and refluorinating and desorbing the neptunium. The development of the latter method is described, and the inherent problems and the effects of variables are discussed.