The objective of this study is first to develop and validate a scientific methodology for the evaluation of diesel engine cold-start performance and then to use this methodology to assess the impact of fuels with respect to two selected fuel properties: variation in the cetane number (from 55 to 70) and use of biodiesel [rapeseed methyl ester (RME) and soybean methyl ester (SME)]. Engine tests were performed at -25 degrees C on a common,rail, four-cylinder diesel engine (Euro 4). The methodology developed shows that the cetane number has a significant impact on the cold-start performance and that the use of cetane improvers ("procetane additives") does not lead to an acceptable performance for a low cetane fuel. The addition of ester has a negative impact on cold operation by increasing the starting delay and opacity. Nevertheless, when the injection settings were simply optimized, the performance obtained in cold conditions with the biodiesel fuel tested here can equal those obtained with a conventional fuel.