This work is focused on the measurement and analysis of the smoke opacity resulting from a Diesel engine fuelled with conventional fuel and biofuels under transient conditions. Methyl esters obtained from used cooking and unused vegetable oils were tested as diesel fuels, pure and blended with 30% and 70% of a commercial diesel fuel which was also used pure. A commercial engine was mounted in a test bench prepared for operating in different transient conditions. A smoke meter AVL 439 allowed for the study of the effect of these fuels on the smoke opacity under varied operating conditions. The thermo chemical properties of the test fuels and the engine parameters, such as fuel/air ratio or exhaust gas recirculation (EGR) ratio, were used for the analysis and interpretation of the results. The engine transient processes studied were (a) engine start, (b) load increase at constant engine speed and (c) engine speed decrease at constant torque. These results suggested that the use of the diesel blends containing vegetable esters is an interesting alternative for a significant reduction in smoke opacity not only in steady conditions but also in transient engine operation, the latter being the most usual condition in passenger vehicles. (c) 2006 Elsevier Ltd. All rights reserved.