Dioxomolybdenum(VI) complexes of general formula [MoO2X2L2] (X=Cl, OSiPh3; L-2 =2-(1-butyl-3-pyrazolyl)pyridine, ethyl [3-(2-pyridyl)-1-pyrazolyl]acetate) were prepared and characterised by H-1 NMR, IR and Raman spectroscopy. The assignment of the vibrational spectra was supported by ab initio calculations. A single crystal X-ray diffraction study of the complex [MoO2Cl2{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}] showed that the compound is monomeric and crystallises in the tetragonal system with space group P4(1). The four complexes are active and selective catalysts for the liquid-phase epoxidation of olefins by tert-butylhydroperoxide. Selectivities to the corresponding epoxides were mostly 100% (for conversions of at least 34%) for the substrates cyclooctene, cyclododecene, I-octene, trans-2-octene and (R)-(+)-limonene. For styrene epoxidation. the corresponding diol was also formed in significant quantities. The turnover frequencies for cyclooctene epoxidation at 55 degrees C were around 340 mol mol(Mo)(-1) h(-1) for the chloro complexes and 160 mol mol(Mo)(-1) h(-1) for the triphenylsiloxy complexes. The addition of co-solvents (1,2-dichloroethane or n-hexane) had a detrimental effect on catalytic activities. Kinetic studies for the two complexes bearing the ligand ethyl[3(2-pyridyl)-1-pyrazolyl]acetate revealed an apparent first order dependence of the initial rate of cyclooctene conversion with respect to cyclooctene or oxidant concentration.