Some fuels with the same research octane number (RON) have different HCCI engine performance. Therefore RON alone cannot be used for determining auto-ignition in HCCI combustion. The current research focuses on creating an HCCI fuel index suitable for comparing different fuels for HCCI operation. More thorough studies are needed to map the fuel effects. One way to characterize a fuel is by using the Auto-Ignition Temperature (AIT). The AIT and the amount of Low Temperature Heat Release (LTHR) together describe the auto-ignition properties of the fuel. Both can be extracted from the pressure trace. The assumption is that the pressure and temperature are known at inlet valve closing (IVC) and that the mass in the cylinder does not change after IVC.The purpose of this study was to map the AIT of different Primary Reference Fuels (PRF) for HCCI combustion at different cylinder pressures. Different pressure levels were achieved by changing inlet air temperatures in 5 steps from 50°C to 150°C. A Cooperative Fuel Research (CFR) engine with variable compression ratio was used. The compression ratio was varied from 5.5 to 15.5 to keep combustion phasing, defined as 50% of total heat released, constant at 3±1° after TDC. The experiments were carried out in lean operation with a constant equivalence ratio of 0.33 and with a constant engine speed of 600 rpm.The results showed that the AITs of the PRFs ranged from around 580 K for PRF 0 to up to 800 K for PRF 100. Auto-ignition was defined as the point where the rate of heat released had reached 0.2 J/CAD. At the lowest inlet air temperatures all fuels except PRF 100 showed Low Temperature Heat Release (LTHR) from about 3% LTHR/Total Heat Release for PRF 95 with up to 19% for PRF 0. Amount of LTHR was found to decrease linearly with increasing octane rating.The low octane PRFs ignited at almost the same temperature independent of the cylinder pressure. The high octane number PRFs (PRF 95 - 100) displayed a wide range of auto-ignition temperatures resulting from the different inlet air temperatures. PRF 80, PRF 85 and PRF 90 showed an intermediate behavior. A constant auto-ignition temperature was seen at the low inlet air temperature but at higher inlet air temperatures the IAT quickly raised when the LTHR disappeared.