The chemical influence of CO and H-2 addition on the auto-ignition of n-heptane/air mixtures is investigated numerically. This work is motivated by the need for a precise control of auto-ignition during the compression stroke of Homogeneous Charge Compression Ignition (HCCI) engines. The numerical simulations are performed using three detailed mechanisms for n-heptane in order to ensure that the results are mechanism independent. The reaction mechanisms used are (i) the Lawrence Livermore National Laboratory (LLNL) version -1 ( 1998) as proposed in (Curran et al., 1998) (ii) an updated recent version of the LLNL reaction mechanism ( Curran et al., 2002) cited as LLNL-2004 in the text (iii) and the detailed mechanism developed by the DCPR ( Buda et al., 2005). The parameters of the simulations are the mixture temperature, pressure, equivalence ratio and the amount of CO and/or H-2 added to the initial charge. It is shown that CO and H-2 influence the auto-ignition delay differently as temperature and equivalence ratio are changed. This is due to the competitions between OH production and destruction. The presence of CO at low temperatures ( T 600 K) lengthens the delay around 5%-10% whereas at high temperatures ( T 1000 K), it shortens the delay in the order of 15%-20%. H-2 addition at low temperatures significantly lengthens the delay whereas at high temperatures (1000 K) does not influence the delay considerably depending on the added amount. A discrepancy is also observed between the results obtained with the LLNL and both the LLNL-2004 and the DCPR mechanisms. This is explained in terms of differences in the kinetic constants of key reactions when CO/H-2 are present in the fresh charge.