The aim of the work was to study the important issue of the residual gas mixing variations, using CO2 as a tracer for the residual gas fraction. Therefore simultaneous measurements of CO2 concentration and temperature have been performed inside a fired IC engine. Temperature is deduced from a broadband N2 CARS signal and the CO2 concentration from a narrowband CO2 CARS signal. To overcome the main difficulties encountered when making measurements in an engine (i.e. cycle-to-cycle variations and beam steering by density gradients), the shot-by-shot referencing technique and the in-situ referencing technique using the N2 CARS signal intensity to determine CO2 concentration were used.All the measurements were performed at the center of the combustion chamber, using different spark plug locations. Three locations were measured: 4cm, 0.5cm and 0.1cm from the plug. The first measurements were performed with three conventional spark plugs, regularly spaced around the combustion chamber. The CARS probe volume was about 4 cm away from the ignition kernels. Using a spark plug with extended electrodes, measurements were then performed at 1 mm and 5mm from the spark location, respectively.Various correlations were studied, the measured correlations range from 0.486 to - 0.272. A positive correlation was found between CO2 concentration and temperature which indicates a nonuniformity in the mixture between the fresh gas and the residual gas. The correlations between CO2 concentration prior to ignition and cycle-to-cycle variations of pressure were studied. Correlations found between the concentration and the maximum pressure, and those between CO2 concentration and the crank angle of maximum pressure, indicate that the residual gas fraction influences the combustion processes during ignition and flame development.