The complexes Cp'Mo(CO)(2)(eta(3)-C3H5) [Cp' = eta(5)-C5H5 (1), eta(5)-C5H4Me (2), eta(5)-C5Me5 (3)] have been prepared, structurally characterised by X-ray diffraction (2,3), and tested as catalyst precursors for the epoxidation of olefins at 55 degrees C. Complex 1 gave a turnover frequency (TOF) of 310 mol mol(Mo)(-1) h(-1) in the epoxidation of cis-cyclooctene with tert- butylhydroperoxide (TBHP, in decane) as oxidant, and 1,2-epoxycyclooctane was obtained quantitatively within 6 h. A similar result was obtained for complex 2, while the TOF for 3 was about one order of magnitude lower, suggesting a possible activity dependence on the ring substituents. For 1 the use of 1,2-dichloroethane as solvent increased the initial reaction rate to 361 mol mol(Mo)(-1) h(-1), with no decrease in epoxide selectivity. Under these conditions the reaction rates for other olefins increased in the order 1-octene < trans-2-octene < cyclododecene < (R)-(+)-limonene < cis-cyclooctene, and, with the exception of limonene, the corresponding epoxide was the only product. For 1 the selective epoxidation of cis-cyclooctene could also be achieved in aqueous solution, using TBHP or H2O2 as oxidants, which gave epoxide yields of 99% and 27% at 24 h, respectively. The possibility of facilitating catalyst recycling by using ionic liquids as solvents was investigated.