Relative rate constants have been determined for the reactions of H atoms and HO2 radicals with toluene and ethylbenzene by adding traces of these compounds to mixtures of H-2 + O-2 at 773 K. The values k(24t) = (5.5 +/- 1.5) x 10(4) and k(24e) = (1.65 +/- 0.63) x 10(5) dm(3) mol(-1) s(-1) are the first reliable kinetic data obtained for the abstraction of an H atom from any aromatic compound by HO2 radicals. It is shown that the values are significantly lower than expected on the grounds of enthalpy of reaction, and it is concluded the explanation lies in a combination of a lower A factor than observed with alkanes (because of the loss of entropy of activation in the emerging electron-delocalized radicals), and a slightly higher activation energy. A comprehensive database for HO2 abstraction reactions from alkanes, aromatics, alkenes, and related compounds has been assembled and recommended for use over the temperature range 600 to 1200 K. HO2 + C6H5CH3 --> H2O2 + C6H5CH2 (24t) HO2 + C6H5C2H5 --> H2O2 + C6H5CHCH3/C6H5CH2CH2 (24e) The overall rate constants k(2tw) = (5.0 +/- 1.0) x 10(8) and k(22e) = (1.07 +/- 0.25) X 10(9) dm(3) mol(-1) s(-1) are obtained for H attack on toluene and ethylbenzene at 773 K. As found for the HO2 reactions with toluene and ethylbenzene, the values are lower than expected, despite the fact that at least one other pathway besides abstraction is known to occur. H + C6H5CH3 --> products (22t) H + C6H5C2H5 --> products (22e)