The hydrated electron and H-atom reduction of aqueous benzaldehyde as well as 2-, 3-, and 4-chlorobenzaldehyde has been studied by pulse radiolysis. The electron attachment occurs on the aldehyde group. In alkaline solutions the radical anions C6H5CHO.- and ClC6H5CHO.- are formed by reaction with e(aq)(-) as well as with (CH3)(2)(COH)-C-. and CO2.- radicals. In acid solution the protonated forms of these anion radicals, C6H5-CHOH. and ClC6H5CHOH., are obtained, pK = 8.2-8.5. The rate constants for the reaction with e(aq)(-) have been determined to be k(e(aq)(-) + C6H5CHO) (2.4 +/- 0.2) x 10(10) dm(3) mol(-1) s(-1) and k(e(aq)(-) + ClC6H9CHO = (2.2 +/- 0.2) x 10(10) dm(3) mol(-1) s(-1) for all three isomers. The electron transfer process from (CH3)(2)(COH)-C-. and CO2.- to the benzaldehydes is much slower, k = (0.7-2.2) x 10(8) dm(3) mol(-1) s(-1) and k = (5.4-16) x 10(8) dm(3) mol(-1) s(-1), respectively, depending on the position of the halide. The anion radicals of the chlorobenzaldehydes undergo neither intramolecular electron transfer nor dehalogenation. This is confirmed by gamma-radiolysis experiments, where neither chloride nor HCHO could be detected as final products. The carbonyl radicals decay by bimolecular processes, 2k = (1.8-2.6) x 10(8) dm(3) mol(-1) s(-1) at pH 10 and 2k = (0.7-1.5) x 10(9) dm(3) mol(-1) s(-1) at pH 3.5. The reactions of H atoms with benzaldehyde and 2,- 3-, and 4-chlorobenzaldehyde, k(H+substrates) = (0.8-1.4) x 10(9) dm(3) mol(-1) s(-1), yield cyclohexadienyl radicals, which decay in bimolecular reactions, 2k = (2.5-3.0) x 10(9) dm(3) mol(-1) s(-1). The absorption spectra of the carbonyl radicals and carbonyl radical anions as well as that of the H-adducts of each individual substrate were measured in the range 290-500 nm and their spectroscopic characteristics determined.