Detailed kinetic data are reported for the reactions of M(CO)6 (where M=Cr, Mo, and W) with Me3NO, in the absence and in the presence of triphenylphosphine (PPha). The rates of reaction are first-order in concentrations of M(CO)6 and of Me3NO but zero-order in PPh8 concentration. The rates of reaction decrease in the order W>Mo>Cr. A mechanism is proposed which involves attack on a carbonyl carbon with the formation of coordinatively unsaturated intermediates of the type M(CO)5, which then rapidly react with an entering ligand. Compared with other nucleophiles reported to react by carbonyl attach in M(CO)6 substrates, Me3NO is a strong nucleophile. Qualitatively, the nucleophilic strengths decrease in the order MeLi>Me3NO>PhCH2MgBr>>N3->NCO->NCS->C1->Br->I-.