Reaction of either (n-Bu4N)(2)[Mo5O13(OMe)(4)(NO){Na(MeOH)}]. xMeOH or (n-Bi4N)(3)[Mo6O18(NO)] with various reducing agents, including N2H4 .2HCl, in methanol or in a mixture of methanol and acetonitrile, yields reduced nitrosyl decamolybdates, among which (n-Bu4N)[Mo10O25(OMe)(6)(NO)] ((n-Bu4N)II) and two forms of (n-Bu4N)(2)[Mo10O24(OMe)(7)(NO)] ((n-Bu4N)(2)IVa and (n-Bu4N)(2)IVb) have been crystallographically characterized. IVa,b are diastereoisomers that differ in the location of methoxo groups. The molecular structures of II and N are closely related to that of [W10O32](4-), the so-called decatungstate Y, and consist of two halves of five edge-sharing octahedra connected through four quasi-linear Mo-O-Mo bridges. Besides the four electrons essentially residing at the Mo(ll) center bearing the nitrosyl ligand, II and IV further accommodate two and four delocalized "blue" electrons, respectively. II and IV thus contain molybdenum atoms in three different oxidation states, Mo(II), Mo(V), and Mo(VI). On the basis of their optical spectra, they are best described as class II mixed-valence complexes according to the classification of Robin and Day. Their intimate electronic structure has been further investigated by means of extended Huckel calculations on the model compound [Mo10O26(OH)(6)]. The composition of the HOMO clearly demonstrates that the "blue" electrons are circulating among the eight equatorial molybdenum sites, the delocalization being strongly favored by the quasi-linear M-o-Mo bridges.