An analytical technique has been optimised for the measurement of the concentrations of diesel exhaust odorants. Application of this technique to combustion bomb studies shows that preflame reactions with diesel fuel produce high concentrations of odorants. The effects of engine variables on exhaust odorant concentrations are presented for direct and indirect injection engines. Analysis of these data shows that diesel exhaust odorants are produced from three sources: (a) the fuel-lean mixture produced during the ignition delay period, (b) fuel emptying from the nozzle sac volume of direct injection engines after injection, (c) a fuel-rich source which becomes significant at high load. The practical measures for control of odorants are outlined.