The molybdenum(II) tricarbonyl complexes [Mo(CO)(3)I2L (n) ] (n = 1, L = 2,2'-bipyridine, 4,4'-di-tert-butyl-2,2'-bipyridine; n = 2, L = pyridine, 4-tert-butylpyridine) have been examined as catalyst precursors for the epoxidation of the biorenewable olefin methyl oleate with tert-butylhydroperoxide. In situ oxidative decarbonylation of the precursors gives highly active and selective molybdenum(VI) catalysts, which were identified as the one-dimensional molybdenum oxide/bipyridine polymer [MoO3(2,2'-bipyridine)], octanuclear [Mo8O24(4,4'-di-tert-butyl-2,2'-bipyridine)(4)], and the pyridinium beta-octamolybdates (LH)(4)[Mo8O26] for L = pyridine or 4-tert-butylpyridine.