A central step in the low-temperature oxidation of hydrocarbons is the abstraction of H by the hydroperoxyl radical (HO2). In this study, reaction rate constants are derived theoretically for H abstraction by HO2 from the three distinct locations of H in ethylbenzene (primary, secondary and aromatic H, with H on the ortho carbon taken as an example of unreactive aromatic H) as well as for the addition of HO2 at the four possible aromatic sites. Potential energy surface is mapped out based on the results of computations performed with the composite CBS-QB3 theoretical method. Rate constants are fitted to modified Arrhenius forms in the wide temperature range of 300-2000 K. The dominant channel at all temperatures is found to be H abstraction from the secondary C of the ethyl chain with a rate constant expression of k = 1.28 x 10(-24)T(3.70) exp (-5100/T) cm(3) molecule(-1) s(-1). Abstraction from the primary C also contributes significantly at higher temperatures (>800 K) with a rate constant expression of k = 2.90 x 10(-24)T(3.78) exp (-8400/T) cm(3) molecule(-1) s (1) Reasonable agreement was obtained with the limited experimental data available in the literature. Addition at the four sites of the aromatic ring and abstraction of one of the C-H aromatic bonds are relatively unimportant over the temperature range studied. We also investigate the abstraction of H from the secondary C on the ethyl chain by triplet oxygen and report the associated rate expression. The results presented herein should be useful in modelling the oxidation of alkylbenzenes at lower temperatures. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved,