Abstract In order to trace the behavior of neptunium by measuring the γ-activities of 237Np and 239Np, two methods for carrying down protactinium-coprecipitation with manganese dioxide and sorption on silica gel-were applied to remove the disturbing γ-emitter, 233Pa. The coprecipitation of 233Pa on manganese dioxide from a nitric or perchloric acid solution is highly effective and ensures efficient separation from neptunium, which remains quantitatively behind in the supernatant solution. This technique is suitable for removing 233Pa from every sample solution just before γ-counting (after the experiment). 233Pa is also effectively sorbed on a silica gel column from strongly acidic solutions of nitric or perchloric acid. Although this technique requires a longer time, it has the advantage of no contamination by foreign ions and is applicable for preparing a 237Np solution free of 233Pa in one lot for use in succeeding experiments.