The compounds MoO(O-2)(2)(alpha-aa)(H2O), alpha-aa = glycine (1), alanine (2), proline (3), valine (4), leucine (5), serine (6), asparagine (7), glutamine (8), and glutamic acid (9), were prepared from the acidic aqueous solutions and characterized by examination of their IR, H-1 and C-13 NMR, and UV-visible spectra. They represent the first complexes containing a peroxo-alpha-amino acid combination in a metal ion ligand sphere. The synthesis and crystallization of these complexes was pH and concentration dependent, and their stability varied for different alpha-amino acids. X-ray structural studies of 1-3 have shown that the alpha-amino acids are coordinated as a zwitterion via one oxygen. This oxygen of the monodentate carboxylato group occupies an equatorial position in a distorted pentagonal bipyramid and encloses the pentagonal ring with the two bidentate peroxo groups. The apical positions are occupied by an oxo group and a water molecule, respectively. The Mo-O(O-2) bonds are nonsymmetrical, differing in length by 0.009-0.045 A. The longer bonds are located next to the coordinated carboxylato oxygen. A correlation between O-O and Mo-O bond lengths with the IR and UV-visible spectra of complexes 1-3 is discussed. Crystal structure of MoO(O-2)(2)(alanine)(H2O) : monoclinic, space group P2(1)/c; Z = 4; a = 10.727(3) Angstrom; b = 8.026(2) Angstrom; c = 10.794(4) Angstrom; beta = 110.81(2)degrees; V = 869 (Angstrom)(3); R = 0.041. H-1 and C-13 NMR spectra in D2O solutions showed the presence of one complex species, which decomposed by standing in solution. Analogies and differences between Mo(VI) and V(V) peroxo complexes are outlined.