The kinetics of the Np(V) disproportionation in 0.1 M potassium hydrogen phthalate + 30 mM EDTA solutions at 25–45°C was studied by spectrophotometry. The reaction rate at pH in the range 3.0–5.0 is described by the equation −d[Np(V)]/dt = k[Np(V)]2, where k = k0[H+]0.65. In solution with pH 4.01 at 25°C, k = 3.39 × 10−2 L mol−1 s−1. The complexes of NpO2+ with HY3− form a dimer, which forms an activated complex with the H+ ion. Decomposition of the activated complex yields Np(IV) and Np(VI). Neptunium(VI) bound with Y undergoes intramolecular reduction to Np(V).