Issue addressed Australians' use of cannabis has been increasing. Over a third of Australians (35.4%) have used cannabis at some time in their lives and 10.3% are recent users. Almost two‐thirds of cannabis users combine cannabis with tobacco. The aim of this study was to understand the process of mulling - smoking tobacco and cannabis together - using a grounded theory approach. Methods Twenty‐one in‐depth semistructured interviews were conducted with men aged 25-34 and living on the North Coast of New South Wales. Interviews explored participants' smoking practices, histories and cessation attempts. Results A model describing mulling behaviour and the dynamics of smoking cannabis and tobacco was developed. It provides an explanatory framework that demonstrates the flexibility in smoking practices, including substance substitution - participants changed the type of cannabis they smoked, the amount of tobacco they mixed with it and the devices they used to smoke according to the situations they were in and the effects sought. Conclusion Understanding these dynamic smoking practices and the importance of situations and effects, as well as the specific role of tobacco in mulling, may allow health workers to design more relevant and appropriate interventions. So what? Combining tobacco with cannabis is the most common way of smoking cannabis in Australia. However, tobacco cessation programmes rarely address cannabis use. Further research to develop evidence‐based approaches for mull use would improve cessation outcomes.