The constant volume combustion apparatus, known as the Ignition Quality Tester (IQTTM), is increasingly used around the world to measure the ignition delay (ID) of compression ignited fuels, and to calculate the derived cetane number (DCN) from correlations developed with the cetane number (CN). The IQTTM is widely used in research due to its capability to manipulate combustion controller parameters, including pressure, temperature, oxygen content and mass of injected fuel. Recently, the IQTTM has also been used to conduct experiments on fuels with a longer I D, such as gasoline. Most findings from previous research are based on instantaneous global pressure measurements to obtain ignition delay, whereas there are minimal spatial and temporal temperature measurements inside the combustion chamber of the IQTTM. This study will expand knowledge of combustion processes through temperature measurements, using a set of fine gauge thermocouples distributed at different locations inside the IQTTM combustion chamber. In this study, the temperature was recorded at 46 locations inside the IQT chamber during fuel injection and combustion. The experiments were performed at the lab of Advanced Engine Technology Ltd. - Ottawa. Two sets of experiments were conducted: injection fuel into nitrogen (no chemical reaction or combustion), and injection fuel into air (chemical reaction or combustion) an the range of ambient air temperatures and pressures was 530 − 590 o C and 10−21.4 bar, respectively. The temperature variations indicated that the biggest drop in temperature was at the chamber centerline, where most of the larger droplets of the fuel spray moved to. A drop was also found to a lesser degree in the region around centerline, while there was no temperature drop near the chamber wall or tail, which means spray did not reach there. A method developed to estimate the physical and total ignition delay based on the temperature measurements is introduced in this study. The combustion initiation lo- cation was predicted from the results of the ignition delay, and it was determined that during long periods of ID the fuel-air mixture becomes pseudo-homogeneous and ....