The oxo-bridged dimer [Mo2O4(mu(2)-O)Cl-2(pzH)(4)] (1;pzH = pyrazole) exhibits unusually high activity in the liquid-phase catalytic epoxidation of the cyclic olefins cyclooctene and (R)-(+)-limonene under mild conditions and in the absence of additional organic solvents, using tert-butyl hydroperoxide as the oxidant. The complex is stable under the reaction conditions and can be used in further catalytic runs without significant loss of performance. An X-ray crystallographic investigation reveals that 1 has an unprecedented and extremely rare all-cis configuration at each of the MoO2(mu(2)-O)Cl(pzH)(2) cores, which can be understood by considering supramolecular contacts and geometric factors.