[en] Using a special type of on line electromigration measurements of γ-emitting radionuclides in homogeneous aqueous electrolytes free of supporting materials the complex formation reactions of carrier-free ²³⁹Np-Np(V) with acetate and citrate ligands in neutral solutions, T = 298.1(1) K, were studied. In the acetate system, μ = 0.30 (ClO₄^-), stoichiometric stability constants of lgK₁ = 0.96, lgK₂ 0.61, were obtained. In the citrate system reduction of Np(V) was observed. Signs and amounts of individual ion mobilities of the acetato complex ions calculated confirm a hypothesis on existence of a NpO³⁺ cation. (author)