Toluene is often used as a fluorescent tracer for fuel concentration measurements, but without considering whether it affects the auto-ignition properties of the base fuel. We investigate the auto-ignition of pure toluene and its influence on the auto-ignition of n-heptane and iso-octane/air mixtures under engine-relevant conditions at typical tracer concentrations. Ignition delay times tau(ign) were measured behind reflected shock waves in mixtures with air at phi = 1.0 and 0.5 at p = 40 bar, over a temperature range of T=700-1200 K and compared to numerical results using two different mechanisms. Based on the models, information is derived about the relative influence of toluene on tau(ign) on the base fuels as function of temperature. For typical toluene tracer concentrations <= 10%, the ignition delay time tau(ign) changes by less than 10% in the relevant pressure and temperature range. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.