A detailed product analysis was carried out at 773 K when toluene (TOL) and ethylbenzene (EB) were added separately in small amounts to H-2 + O-2 + N-2 mixtures at 500 Torr total pressure. B-enzaldehyde is the major initial product from TOL, formed in reaction (26) and in the overall reaction (3 1). C6H5CH2 + HO2 --> OH + C6H5CH2O (+ O-2 -->C6H5CHO) (26) C6H5CH2 + O-2 --> C6H5CHO + OH (31) The electron-delocalised benzyl radical reacts very slowly with O-2, and the importance of radical-radical reactions is confirmed by the observation that bibenzyl is formed in relatively high yields. Values of k(26) = (5.1 +/- 1.5) X 10(9) dm(3) mol(-1) s(-1) and k(29) = 2.8 X 10(3) s(-1)(for the 1,3 Hatorn transfer involved in (31)) are obtained. C6H5CH2OO --> C6H5CHOOH (29) Styrene,is the major initial product from EB. The electron-localised radical C6H5CH2CH2 reacts almost completely in a fast reaction (k = 2 X 10(8) dm(3) mol(-1) S-1) with O-2 to give styrene, but the more stable electron-delocalised C6H5CHCH3 radicals also undergo radical-radical reactions to give benzaldehyde with k(38) = (7.3 +/- 3.0) X 10(9) dm(3) mol(-1) s(-1.) C6H5CHCH3 + HO2 --> C6H5CH(OOH)CH3 (38) Measurements of the yields of benzene from TOL and EH gave rate constants for the reaction H + TOL/EB --> C6H6+ CH3/C2H5, and combination with independent data gives k = 950T(2)exp(-475) dm(3) mol(-1) s(-1) for the TOL reaction. Rate expressions are given in non-Arrhenius form for all H abstractions by H atoms from TOL and EB. Further evidence is provided that H abstraction from the alpha-carbon atom-in alkyl benzenes is considerably slower than expected on thermochemical grounds. It is, however, concluded that abstraction from the benzene ring by H atoms is significantly more important than hitherto suggested. (C) 2003 The Combustion Institute. All rights reserved.