In this study, direct numerical simulations of the reacting mixing layer between two streams of air and fuel with different velocity and temperature have been performed using a four-step mechanism for the ignition of n-heptane. The localization of the ignition spots and the dependence of the ignition delay time on the initial conditions have been discussed and the results have been compared with those of several experimental and numerical contributions in the literature. The results show the importance of mixture fraction and scalar dissipation rate in favoring the ignition process on the sides of the jet, fully matching the outcomes of other works that use more simplified kinetic mechanisms. This result highlights the role of fluiddynamic phenomena in the development of autoignition in a mixing layer. Furthermore, ignition spots are found on the tip of the jet favored by a local increase in temperature. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.