Rate coefficients for the reactions of hydroxyl (OH) and nitrate (NO3) radicals with benzaldehyde and the tolualdehydes have been determined at 295 ± 2 K and atmospheric pressure using the relative rate technique. Experiments were performed in atmospheric simulation chambers using gas chromatography for chemical analysis. The rate coefficients (in units of cm3 molecule−1 s−1) are:View Within ArticleThe reactivity of the aromatic aldehydes is compared to other aromatic compounds and it is shown that, for the tolualdehydes, the OH and NO3 rate coefficients do not depend on the position of the CH3 substituent on the aromatic ring. The new data are used to show that the gas-phase reactivity of the aromatic aldehydes towards OH and NO3 radicals follows a linear free energy relationship typical of addition reactions, although the net result is H-atom abstraction. The rate coefficient data are explained in terms of known mechanistic features of the reactions and simple theoretical calculations have been performed in an attempt to understand the observed trends in reactivity. The atmospheric implications are also discussed.