This study came about as an attempt to better understand differences in patterns of heroin use. The relative popularity of particular routes of administration of heroin is influenced by the properties and characteristics of heroin itself. Heroin sold on the street is generally available in two forms: as a heroin salt, such as hydrochloride, which is freely soluble in water, or as a heroin free base, which is insoluble in water. Heroin that is injected or snorted takes the form of heroin hydrochloride (HCI) and smoking heroin is usually free base, which is appreciably more volatile than salt. The bioavailability of heroin is determined by a number of factors, including form/composition and route of administration, the presence of adulterants and impurities, and chasing/injecting techniques. To study the purity and composition of heroin in Sydney, a retrospective sampling frame was constructed consisting of all suspected heroin seizures in Cabramatta between October 1996 and March 1997. A total of 33 street-level “exhibits,” comprising 88 samples, were selected. Ion chromatography was used to determine whether heroin was present as the free base or as the hydrochloride. High performance liquid chromatography with diode array detection was used to assess the presence of diacetylmorphine (heroin hydrochloride), 0-6-monoacetylmorphine hydrochloride (degradation product), and acetylcodeine hydrochloride (synthesis product). Gas chromatography/mass spectrometry and high performance liquid chromotography with refractive index detection were used to detect adulterants and diluents. All samples were free of harmful adulterants. Adulterants detected were pharmacologically inactive diluents largely used to add bulk (sugars) or pharmacologically active adulterants used to improve the bioavailability of heroin HCL when smoked (caffeine). Results have implications for attempts to reduce drug-related harms and, in particular, suggest that interventions designed to facilitate transitions from heroin injecting to smoking require consideration of the pharmacological factors associated with the route of administration. Tables, figure, references