The autoignition of C8H10 aromatic/air mixtures (ortho-xylene, meta-xylene, para-xylene, and ethylbenzene in air) has been Studied in a shock tube at temperatures of 941 - 1408 K, pressures of 9-45 atm, and equivalence ratios of phi = 1.0 and 0.5. Ignition times were determined using electronically excited CH emission and pressure measurements. The measurements illustrate the differences in reactivity for the C8H10 aromatics under the studied conditions. Ethylbenzene was by far the most reactive C8H10 aromatic with ignition times a factor of two to three shorter than the xylenes. The xylene isomers exhibited ignition times that were similar, with o-xylene the most reactive, p-xylene the least reactive, and m-xylene just slightly more reactive than p-xylene. The p-xylene ignition times are almost identical to previous measurements for toluene at the same conditions. The differences in reactivity can be attributed to the C-H and C-C bond strengths in the alkyl side chains and the proximity of the methyl groups in the xylenes. These results represent the first ignition measurements for C8H10 aromatics at the elevated-pressure moderate-temperature conditions studied, providing needed targets for kinetic modeling at engine-relevant conditions. Kinetic modeling illustrates the importance of the methylbenzyl + HO2 reaction and indicates further study of this reaction is warranted. (C) 2008 The Combustion Institute, Published by Elsevier Inc. All rights reserved.