~ 4MeCN 3 were prepared. Compound 2 contains the two-electron reduced [ H PMO,,O,]~-ion having the a-Keggin structure, 3 the four-electron reduced [H,S,MO,,O,]~-ion having the CC-Dawson structure. Compounds 1-3 were characterised by means of IR and NMR spectroscopy and X-ray diffraction. The structures of 1 and 2 are disordered in space group P1. The heteropolyanions show apparent T,, symmetry; their actual symmetry is T. Upon reduction the M o Mo distances increase slightly. The strongly alternating 'short' (mean 1.814 A) and 'long' (1.990 A) Mo-0-Mo bonds in the unreduced compound 1 become more equal in 2. The structural changes upon reduction observed in 3 were analysed in detail. The most significant alterations are an increase of the M o M o distances between corner-sharing MOO, octahedra in the equatorial belt by 0.066 A, and a decrease of the Mo-0-Mo bond lengths connecting the two halves of the anion. The alternation of the Mo-O(bridge) bonds is nearly evened out after reduction. All structural changes observed in 2 and 3 are consistent with the description that the additional electrons occupy delocalised molecular orbitals extending over the Mo-0, framework.Reduced heteropolyanions ('heteropoly blues') are of continuing interest in important areas of chemistry like analytical and medical applications, catalysis, electron transfer and delocalisation of electrons in mixed-valence compounds. Despite significant evidence obtained by electrochemical studies, and by UV, ESR and multinuclear NMR spectroscopy, structural data are still scarce. Pope and co-workers5'6 reported the structure of four-electron reduced P-[H,PMo, 2040]4-, Miiller et al. ' the structures of four-electron reduced P-[H,AsMo,,-