A semi-reduced (70 species, 210 reactions) and a skeletal (27 species, 29 reactions) chemical reaction mechanism for iso-octane are constructed from a semi-detailed iso-octane mechanism (84 species, 412 reactions) of the Chalmers University of Technology in Sweden. The construction of the reduced mechanisms is performed by using reduction methods such as the quasi-steady-state assumption and the partial equilibrium assumption. The obtained reduced iso-octane mechanisms show, at the mentioned conditions, a perfect coherence with another more detailed iso-octane mechanism of ENSIC-CNRS (2411 reactions and 473 species) and the semi-detailed iso-octane mechanism of Chalmers. The validity of this mechanism with regard to the ignition delay is determined for several engine parameters adhering to HCCI conditions: inlet temperature (303-363 K), equivalence ratio (0.2-0.7) and compression ratio (10-16). The ignition delay is found to be decreased by an increase in the inlet temperature, a decrease in the equivalence ratio and an increase in the compression ratio. In order to validate the effects of the inlet temperature, compression ratio on the auto-ignition delay, experiments are performed on a CFR engine. A good agreement is obtained between experimental results and calculations. (c) 2005 Elsevier Ltd. All rights reserved.