The oxidation and autoignition of five undiluted stoichiometric mixtures, n-heptane/toluene, isooctane/toluene, isooctane/1-hexene. 1-hexene/toluene. and isooctane/1-hexene/toluene, has been Studied in a rapid compression machine below 900 K. Ignition delay times of two- and one-stage autoignition have been measured and compared to those for pure hydrocarbons. The largest influence of mixing is in the region of the negative temperature coefficient. Intermediate products have been analyzed. The main reaction paths of low-temperature co-oxidation are discussed according to current knowledge of the oxidation paths of pure hydrocarbons. The influence of toluene on the temperature coefficient of the first stage of ignition of isooctane cannot be accounted for by the current theories of low-temperature autoignition. Each hydrocarbon generates a pool of radicals whose reactivity and selectivity toward further attack changes with temperature and with the family of hydrocarbons. The overall behavior of mixtures may result from changing competition for HO2 and OH as temperature increases during the delay time. Termination reactions between stable radicals seem to have a minor impact at low temperature. (c) 2006 Published by Elsevier Inc. on behalf of The Combustion Institute.