The use of biomass fuels such as biodiesel as an alternative fuel for petroleum diesel in automotive sector is of great importance today, as it reduces global warming. Previous research has pointed out that biodiesel/diesel blends can be used in diesel engines with little or no modification. It is estimated that the differences on the characteristics of diesel engines exhaust emissions are due to the different molecule composition of conventional diesel and biodiesel fuels.The scope of this work was to compare the exhaust gas emissions from the use of mixtures of biodiesel and diesel and their effect on the performance of a passenger car engine. The blends B10, B50 and neat biodiesel, B100, were used in an old technology 1600 cc diesel car, equipped with a retrofit catalytic diesel particulate filter. While the car was running on a chassis dynamometer with wide open throttle and under full engine load, measurements of torque, speed and fuel consumption were taken for three different vehicle speeds and gear ratios. Regarding the emissions, the concentrations in the exhaust gases of CO₂, CO, HC, NOX and soot opacity values were recorded. The results clearly showed that increasing the percentage of biodiesel in the fuel, adversely affected the emissions of NOX as compared with neat biodiesel. Diesel engines always run "lean," thus the emissions of CO and HC were very low and practically negligible. On the other hand, CO₂ emissions primarily depend on the raw materials and the processes that are used for the production of biodiesel. Soot emissions decreased when blends with higher biodiesel content were used. The engine performance, as measured on the chassis dynamometer, increased slightly by the use of B10, B50 and B100 fuels. On average, the torque increased by 4.6% and the power by 3.1%, but these results were obtained along with an 11.5% higher fuel consumption. The best overall results, at all speeds, were obtained using the B100 fuel, with a remarkable average reduction of 26% in soot emission.