Synthesis of the dioxomolybdenum(VI) complexes [MoO₂(L^3OMe)(EtOH)] (1), [MoO₂(L^4OMe)(EtOH)] (2) and [MoO₂(L^H)(EtOH)] (3) and dioxotungsten(VI) complexes [WO₂(L^3OMe)(EtOH)] (4), [WO₂(L^4OMe)(EtOH)] (5) and [WO₂(L^H)]_n (6a) was carried out using [MO₂(C₅H₇O₂)₂] (M = Mo or W) and the corresponding aroylhydrazone ligand H₂L^R (3-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H₂L^3OMe), 4-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H₂L^4OMe), or salicylaldehyde 4-hydroxybenzhydrazone (H₂L^H) in ethanol. Compounds obtained upon heating of the mononuclear complexes in acetonitrile or dichloromethane, [MO₂(L^R)]_n (1a–6a) or [MoO₂(L^3OMe)]₂ (1b), respectively, were also investigated. Crystal and molecular structures of the mononuclear 1, 2 and 3, polynuclear 1a·MeCN and dinuclear 1b complexes were determined by the single crystal X-ray diffraction method. Powder X-ray diffraction showed isostructurality of 1 and 4, and 2 and 5. The complexes were further characterized by elemental analysis, IR spectroscopy, TG and DSC analyses, and one- and two-dimensional NMR spectroscopy. The catalytic performances of 1–5 and 6a were investigated for epoxidation of cyclooctene using aqueous tert-butyl hydroperoxide (TBHP) as the oxidant.