Since 1995 heroin sample comparisons have been carried out in Viet Nam to establish links between wholesalers and retailers. To that end, the physical and chemical characteristics of samples are analysed: their colour, the packaging material, including fingerprints, diacetylmorphine (heroin) content and the composition of some main alkaloids. At the beginning of 2002, having acquired expertise on impurity profiling and with the support of new instruments, the Institute of Forensic Sciences of Viet Nam introduced the routine impurity profiling of seized heroin and methamphetamine and later undertook to explain that process to national law enforcement bodies. Since then, 375 heroin and 29 methamphetamine samples have been analysed for major and minor impurities. Substances detected in the analysis of illicit heroin include diacetylmorphine, morphine, codeine, 06-monoacetylmorphine and acetylcodeine as well as adulterants such as paracetamol and caffeine. Since methamphetamine impurity profiling began, 29 samples have so far been analysed, and some samples have been grouped through the application of cluster analysis. In the case of heroin, impurity profiling has established a link between two major trafficking groups suspected of obtaining heroin from the same source of production. Analysis has also revealed a link between one wholesaler and several retailers in one region. In addition, impurity profiling provides new information on the preparation and production of some methamphetamine and fake Ecstasy tablets.