[en] Using as an example several reactions of neptumium, the spectrophotometric method was employed to study the effect of ''neutral'' (Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺, La³⁺) and ''active'' (UO₂²⁺, In³⁺) cations on the rate of oxidation-reduction reactions in which NpO₂ ions are reduced as far as Np⁴⁺ state or are formed from NpO₂²⁺ to Np⁴⁺ ions. An equation was obtained which quantitatively describes the ''salt effect''. Dependence was established between the parameters of this equation and the nature of the cations added. For reactions in which NpO₂⁺ ions participate as the cation-reactant, in the presence of ''active'' cations the change in the rate of oxidation-reduction reaction is caused not only by the ''salt effect'' but also by the transition of NpO₂⁺ ions from a free state to the ''bound'' state. Equations were obtained describing the dependence between the equilibrium constant for reversible reactions involving NpO₂⁺ ions and the concentration of ''neutral'' and ''active'' cations