The reactions of the oxidizing radicals hydroxyl radical (·OH), azidyl radical (N3.) and sulfate radicalanion (SO4—.) with hydroxy-, methoxy-, hloro- and nitro-substituted benzaldehydes were studied by pulse radiolysis.The rate constants for the.OH reaction [k(2.6≈12.0)×1091mol-1s-1] with these compounds are higher than those for the N3.or SO4.reactions [k(0.01–4.2)×109 lmol-1s-1]. The.OH reaction shows a significant substituent effect which is reflected in the rate constants (k≈12.0×109dm3mol-1s-1with hydroxybenzaldehyde and k≈3.0×109dm3mol-1s-1with nitrobenzaldehyde). The transient species produced by the reaction of .OH with them -substituted benzaldehydes shows an absorption maximum at 370–400 nm, whereas the p-substituted isomers exhibit two absorption peaks centred around 325 and 410 nm, except in the case of p-nitrobenzaldehyde, which shows only a single peak at 330 nm. These spectra are identical with the spectra obtained for the N.3or SO.4.reactions with substituted benzaldehydes..OH adduct formation and subsequent decomposition to the corresponding phenoxylradicals is the main reaction channel for the.OH-induced oxidation of substituted benzaldehydes. One-electronoxidation by N3.or SO4.of substituted benzaldehydes proceeds via an electron transfer process producing the corresponding radical cations.