Abstract Oxygen-induced 1,4-addition reactions of trialkylboranes with cyclic α, β-unsaturated carbonyl compounds were studied. Stereoselective 1,4-additions of the alkyl groups of trialkylboranes were observed in the reactions of d-carvone and l-perillaldehyde. Namely, it was shown that the alkyl group adds to d-carvone from the trans side with respect to isopropenyl group to give the corresponding saturated ketones. The same type of one side attack of the alkyl group to l-perillaldehyde was also observed. The evidence seems to strongly support the hypothesis that coordination of boron atoms of trialkylboranes to carbonyl oxygens plays an important role, even in the oxygen-induced free radical chain reaction of organoboranes with α, β-unsaturated carbonyl compounds.