Carbonyl complexes of formula CP'M(CO)(3)R (M = Mo, W; R = alkyl) can be applied as very active (TOFs up to ca. 6000 h(-1); TON > 500) and selective olefin epoxidation catalysts. The X-ray crystal structure of one of those complexes is reported. The carbonyl compounds are in situ oxidized by tert-butyl hydroperoxide (TBHP) and transformed to the catalytic active species without loss of the Cp'- and R-ligands. Mocomplexes are far more active than W compounds, increasing alkylation of the Cp ligand lowers the catalytic activity. Ansa-bridged derivatives, being more difficult to synthesize than unbridged congeners, show no significant advantage when applied in catalysis. The catalysts lose activity after several runs, mainly due to competitive coordination of the increasing presence of the by-product t-BuOH. (C) 2004 Elsevier B.V. All rights reserved.