Stability and coordination of neptunyl(VI) and -(V) acetate complexes in aqueous solution were studied by using UV-vis-near-IR (NIR) and X-ray absorption fine structure (XAFS) spectroscopy. In the neptunyl(VI) acetate system, the formation of Np(VI)O(2)(AcO)(+), Np(VI)O(2)(AcO)(2)(aq), and Np(VI)O(2)(AcO)(3)(-) was detected. Both spectroscopic methods provided similar stability constants: log K(1) = 2.98 +/- 0.01, log beta(2) = 4.60 +/- 0.01, and log beta(3) = 6.34 +/- 0.01 from UV-vis-NIR and log K(1) = 2.87 +/- 0.03, log beta(2) = 4.20 +/- 0.06, and log beta(3) = 6.00 +/- 0.01 from XAFS at I = 0.30 M (H,NH(4))ClO(4). Extended XAFS (EXAFS)-derived structural data for Np(VI)O(2)(2+)(aq), Np(VI)O(2)(AcO)(+), and Np(VI)O(2)(AcO)(3)(-) were consistent with their stoichiometry, showing a bidentate coordination of acetate (Np-O(ax) = 1.76-1.77 A; Np-O(eq) = 2.43-2.47 A; Np-C(c) = 2.87 A; Np-C(t) = 4.38 A). Similar to Np(VI), Np(V) forms also three different complexes with acetate. The stability constants of Np(V)O(2)(AcO)(aq), Np(V)O(2)(AcO)(2)(-), and Np(V)O(2)(AcO)(3)(2-) were determined by UV-vis-NIR titration to log K(1) = 1.93 +/- 0.01, log beta(2) = 3.11 +/- 0.01, and log beta(3) = 3.56 +/- 0.01 at I = 0.30 M (H,NH(4))ClO(4). The present result is corroborated by the structural information from EXAFS (Np-O(ax) = 1.83-1.85 A; Np-O(eq) = 2.51 A; Np-C(c) = 2.90-2.93 A) and by the electrochemical behavior of the Np(V/VI) redox couple in the presence of AcOH as a function of the pH.