The relative reactivity of the xylene isomers has been compared under preignition and autoignition conditions. Preignition experiments of each isomer with n-dodecane were conducted in a flow reactor facility at 600-850K temperatures, 0.8MPa pressure, and 0.23-0.30 equivalence ratios. Stable intermediates identified from xylene oxidation were tolualdehyde, toluene, and cresol, and the xylenes showed similar reactivity. Autoignition experiments of each isomer with n-decane were conducted in a single-cylinder research engine, and o-xylene was the more reactive isomer. Among several proposed JP-8 surrogates containing xylene, a six-component surrogate with m-or p-xylene was found to best approximate the autoignition conditions of average JP-8. The data can be used for the improvement of xylene chemical kinetic models, and the conclusions from this study will aid in the selection of the appropriate xylene isomer for JP-8 surrogate fuels.