Unique current-time relations were observed during the controlled potential electrolysis for the reduction of NpO(2)(+) to Np(4)(+) or Np(3+) at a gold or platinum electrode in perchloric acid solution. The reduction process of NpO(2)(+) was elucidated based on controlled potential electrolysis and voltammetry as follows: Though NpO(2)(+) is not reduced directly at a gold electrode, it is reduced by chemical reaction with Np(3+) which is produced by the electrode reduction of Np(4+). Here, Np(4+) is initially produced through the disproportionation of NpO(2)(+), and generated progressively through the reduction of NpO(2)(+). The reduction of NpO(2)(+) at a platinum electrode is initiated by the electrocatalytic reduction to Np(4+) with the hydrogen atom which is produced by the electrode reduction of H(+) and adsorbed on the platinum electrode surface. The Np(4+) produced functions as an electron transfer mediator to reduce NpO(2)(+) similar to the reaction at a gold electrode. The above described catalytic reduction processes of NpO(2)(+) were elucidated with the aid of digital simulation of bulk electrolyses. Methods useful for the preparation of Np(4)(+) and Np(3)(+) by bulk electrolysis were proposed based on the experimental results. (C) 2009 Elsevier B.V. All rights reserved.