Biodiesel use as an automotive fuel is expanding around the world and this calls for better characterisation of its impact on diesel combustion, and emissions. A neat soybean-oil derived biodiesel (B100) and its 50 vol.% blend with petroleum diesel (B50) were used on a Euro 2 diesel passenger car. Measurements of CO(2),CO, HC, NO(x), PM, particle number and size distribution and carbonyl compounds (aldehydes and ketones) were conducted on a chassis dynamometer. Acceleration tests were performed, fuel consumption was calculated and measured and metals concentration in the lubricant was determined, in order to assess the biodiesel impact on engine performance and wear. Tests were conducted over the certification cycle and real-world driving cycles. Results showed that biodiesel presence had a negative impact under cold start conditions on both regulated emissions and fuel consumption. However the picture was different in the case of warm start real-world cycles, where differentiations were limited and in several cases biodiesel had a beneficial effect on emissions and efficiency. Regarding particle number, solid particles decreased with biodiesel but total particles increased. Certain carbonyl compounds were also increased with B100. Overall, biodiesel at high blending ratios may strongly impact emissions, in a rather non-uniform manner, with the actual effect being dependant on driving conditions and blending ratio. Some of the differentiations observed were not expected when compared to similar results presented in the literature. However this study does not reach definitive conclusions but rather presents a case that may appear in the European passenger car fleet, which requires further attention and research. (C) 2009 Elsevier Ltd. All rights reserved.