The redox chemistry of Np(V/VI) was investigated in ∼0.6M tetramethylammonium hydroxide/chloride (TMA-(OH, Cl)) solutions with 9 ≤ −log [H+] ≤ 13.5. Redox conditions were defined by the absence or presence of ClO− as oxidizing agent (Na-salt, 5×10−3M and 5×10−2M). The high total Np concentration ([Np]tot ∼ 2×10−3M) led to the precipitation of solid phases in some of the samples. The carbonate concentration (as impurity of TMA-OH) was 2-3×10−3M. UV-vis/NIR spectra obtained from the supernatant in TMA-(OH, Cl) solutions and absence of ClO− showed clear Np(V) features, identified as NpO2+, NpO2CO3− and (NpO2)x(CO3)y(OH)zx−2y−z. No NIR features were observed within 800nm ≤ λ ≤ 1300nm for samples with ClO−. XANES edge energies and features of these samples confirmed the predominance of Np(V) in the absence of ClO− and Np(VI) in the presence of ClO−, by comparison to XANES reference spectra of Np(III/IV/V/VI) prepared within the present work by in-situ electrolysis. Asimilar Np redox distribution was observed for the solid phases based on XANES and EXAFS measurements. EXAFS spectra indicative of NpVO2OH(s) and NpVIO3· xH2O(s) were obtained for samples in absence and presence of ClO−, respectively. The formation of aNa-Np(VI) phase in 5×10−2M ClO− and −log [H+] ∼ 12 was also indicated from the EXAFS, chemical analysis and SEM-EDS. These results indicate that Np(VI) aqueous species and solid compounds prevail far below the oxidation border of water in alkaline solutions and also far below the EH border calculated with the current NEA data selection [1]. These observations are further supported by correlations of literature thermodynamic data for actinides (U, Np, Pu and Am), which predict the formation of NpO2(OH)3− and NpO2(OH)42− aqueous species with stability constants (log *βº1,3 and log *βº1,4) similar to those available for U(VI
