A KrF excimer laser system is experimentally used to photochemically induce the reduction of 1 × 10−4 mol/dm3 Np(VI) to Np(V) while in a solution of 30% tri-n-butyl phosphate (TBP) and nitric acid. It is found that Np(VI), initially prepared from KMnO4 and then extracted by 30% TBP, can be reduced by the reaction with nitrous acid, which is produced by photolysis of nitric acid. The experimental results have demonstrated that laser-induced redox reactions are effective in changing the distribution ratio and reaching equilibrium quickly.