Reaction of solvent substituted MoO2X2(Solv)(2) complexes ((Solv)= THF, CH3CN) with mono- and bidentate nitrogen and oxygen donor ligands leads to complexes of the type MoO2X2L2 in nearly quantitative yields at room temperature within a few minutes. The Mo-95 and (1)7O NMR data of selected complexes as well as the Mo=O LR vibrations were used to probe the influence of the ligands on the electronic properties of the metal and the Mo=O bond. Two complexes have additionally been examined by single crystal X-ray analysis. The activity of the MoO2X2L2 complexes as catalysts in olefin epoxidation with t-butylhydroperoxide as oxidizing agent depends on both the nature of the organic ligand L and the halogeno ligand X. The difference in activity observed between Cl and Br substituted complexes is not very pronounced. In general, the Cl derivatives are more active than their Br analogues. The organic ligands L display a significant influence on the catalytic performance. Complexes with ligands bearing aromatic substituents at N are in all cases, much more active than those bearing aliphatic substituents, The less active complexes can be activated by raising the temperature and extending the reaction time. In all observed cases, these changes produce a significant increase of the product yield.