The goal of this study is to find out the exhaust emissions differences produced by different kinds of fatty acid methyl esters (FAME) derived from used cooking oils and animal fats, as well as the importance of the purification step in exhaust emissions production. A total of 120 L of waste vegetable oil and 30 L of waste frying oil were collected and converted into three batches of FAME. There were two batches of FAME produced from waste vegetable oil (B01 and B02), and one batch of FAME produced by mixing 2% of waste frying oil with waste vegetable oil (B03). The FAMEs used in this study had higher density, kinematic viscosity, and flash point, but a lower gross heating value, when compared to the premium diesel. The B01 engine produced higher CO formation and the diesel-fuelled engine produced higher CO than the B02 and B03 did for engine speeds higher than 1400 rpm. Most of the FAME fuels produced higher CO2 than the diesel fuel did. The FAME fuels emitted higher NOx and PM, but lower SO2, than the diesel fuel. CnH2n+2, diphenyl sulfone (C12H10O2S), and diethyl phthalate (C12H14O4) can be selected as the character index for the combustion of FAME. (c) 2007 Elsevier Ltd. All rights reserved.