An experimental and theoretical study about the autoignition phenomenon has been performed in this article. A new procedure to predict ignition delays under transient (i.e. variable) thermodynamic conditions has been developed starting from the Muller's chemical kinetics mechanism. The results obtained have been compared with those obtained from the Livengood & Wu integral method, as well as with direct chemical kinetic simulations. All simulations have been performed with CHEMKIN, employing a detailed chemical kinetic mechanism. The simulations have been validated in the working range versus experimental results obtained from a Rapid Compression-Expansion Machine (RCEM). The study has been carried out with n-heptane as a diesel fuel surrogate. The experimental results show a good agreement with the direct chemical kinetic simulations. Besides, better predictions of the ignition delay have been obtained from the new procedure than the ones obtained from the classic Livengood & Wu expression. (C) 2015 Elsevier Ltd. All rights reserved.