The relative reactivities of a number of α,β-unsaturated ketones used as flavourings were determined using glutathione as the nucleophile. Monosubstitution at the β-position of the α,β-unsaturated system impeded nucleophilic addition by approximately 1000 times. β-Disubstitution reduced reactivity by more than 100,000 times. Endocyclic α,β-unsaturated ketones were generally less reactive than alicyclic analogues. By way of comparison, the most reactive flavouring investigated, 2-octene-4-one, was consumed by glutathione about 700 times less rapidly than was methylvinyl ketone. Methylvinyl ketone was found to condense with guanylic acid 240,000 times more slowly than with glutathione. It is concluded that α,β-unsaturated ketones used as flavourings generally possess low electrophilicity.