The coordination and redox behavior of Np(IV), Np(V), and Np(VI) sulfate in aqueous solution were investigated by Np L-3-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. The sulfate coordination mode, that is, monodentate versus bidentate, was determined by using neptunium-sulfur distances RNp-S and coordination numbers N-S obtained by EXAFS spectroscopy. Np(VI) is coordinated by sulfate in the bidentate (RNp-S = 3.12 +/- 0.02 angstrom) and monodentate (RNp-S = 3.61 +/- 0.02 angstrom) modes at a low sulfate concentration of [SO42-]/[NpO22+] = 1. At higher [SO42-]/[NPO22+] ratios, bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np(VI) with 2.0 M SO42- and at pH 1.1. Np(V) is coordinated by sulfate in the bidentate (RNp-S = 3.16 +/-0.02 angstrom) and monodentate (RNp-S = 3.67 +/- 0.02 angstrom) modes. However, sulfate coordination is less pronounced and does not exceed one SO42- per Np(V) with 2.0 M SO42-. The redox reaction between the Np(VI)/Np(V) couple Gan be basically categorized as quasi-reversible. It becomes a more irreversible character at high sulfate concentrations due to structural rearrangement of the sulfate ligands. Finally, Np (IV) also shows bidentate (RNp-S = 3.06 +/- 0.02 angstrom) and monodentate (RNp-S = 3.78 +/- 0.02 angstrom) coordination modes. The sulfate coordination increases with an increasing [SO42-]/[Np4+] ratio. A comparison of other tetravalent actinides shows that the monodentate sulfate coordination decreases whereas the bidentate coordination increases along the series Th(IV)-U(IV)-Np(IV). This trend was studied by DFT calculations and is discussed in terms of solvation energy and increasing number of unpaired electrons.