One key parameter of the heroin market available for DPMP is the purity of illicit drug seizures made by the Victoria Police and analysed by the Victorian Forensic Sciences Centre. To date the information derived from this purity data has not been systematically examined in Victoria. The aim of this component of the research was to examine the purity series available for heroin and amphetamines in detail, with a view to determining the nature of heroin seizure purity changes in the context of the change in heroin supply known as the heroin ‘drought’ and whether they are sensitive to assumed changes across the chain of drug supply. During the period immediately around the drought we consider the changes in purity at both the wholesale and retail levels and how the markets recovered in the period after the drought. Through comparisons with the supply in the market of methamphetamine we consider the impacts on this market and evidence for one of the possible substitution effects that may have occurred.