Particulate emission from diesel engines is receiving a great deal of attention due to its probable carcinogenic property. In the exhaust pipe of a diesel engine, the change of the exhaust gas temperature can result in nucleation and condensation of volatile materials and coagulation of particulates. The objective of the present study is to investigate experimentally the transformation of diesel particulates within the exhaust pipe when the exhaust gas is being cooled. Using a water cooler to control and adjust the exhaust gas temperature, the influences of exhaust-gas cooling on mass concentration, composition, size distribution and number-averaged diameter of the particulates, under different engine operating conditions, were analyzed. In addition, the effect of the flow rate of the exhaust gas in the cooler on the number-averaged diameter of the particulates was also discussed. The results showed that the transformation of the diesel particulates in the exhaust pipe depended mainly on the level of cooling, the concentration of the volatile materials, the initial concentration of the particulates in the exhaust, and the residence time of the exhaust gas within the exhaust pipe. The mass concentration and the soluble organic fraction of the particulates increased, while the gaseous hydrocarbon concentration decreased upon cooling the exhaust. Coupled with these were a decrease in the total number of particulates and an increase in the number-averaged diameter of the particulates, as measured by the Electrical Aerosol Size Analyzer. The results will be useful in understanding the mechanisms leading to changes in diesel particulates in an exhaust pipe. (C) 2003 Elsevier Ltd. All rights reserved.