Relative reactivities of α,β-unsaturated carbonyl compounds toward n-butylmagnesium bromide are determined by competitive reactions with diethyl ketone. α-Methyl group significantly suppresses the reactivity of the acrylic ester in ether, but enhances the reactivity of the unsaturated nitrile under the same condition. A striking decrease in reactivity is caused by the β-methyl group in crotononitrile. whereas methyl crotonate is only slightly less reactive than methyl methacrylate. α-Methyl and β-methyl sub-stituents in unsaturated ketones behave in a similar way to those in the unsaturated esters. Polar solvents, in general, elevate reactivity of β-methyl compounds relative to α-methyl ones. The homolytic (or heterolytic) character of the Grignard reactions is discussed quantitatively in terms of the relative reactivities of the three homologous (unsubstituted, α-substituted, and β-substituted) unsaturated compounds.