INTRODUCTION: While many attempts have been made to reduce prescribing errors (PEs), they persist. PE is not in itself a behaviour, but a consequence of a prescribing behaviour. Interventions aimed at prescribers should focus on understanding prescribers' behaviours. OBJECTIVES: The aim of this study was to use the capability, opportunity, motivation - behaviour (COM-B) model to explore the behaviours that could have caused PEs made by senior doctors in a speciality paediatric inpatient ward. METHODS: A qualitative approach was used to investigate prescribers' behaviours in a 26-bed paediatric oncology ward. Error data were collected over a two-month period and were presented during focus groups with prescribers, which were audio-recorded and transcribed verbatim. Thematic analysis was used to identify contributory factors to errors, which was used to identify sources of behaviours using the COM-B model. RESULTS: Behaviours related to prescribers' capabilities were: prescribers' improper use of the software because of insufficient skills, and prescribers' inability to prescribe correctly because of lack of knowledge. Behaviours related to opportunities in the environment were: prescribers' inability to make an informed decision because of poor access to patient information, inability to properly complete a task because of heavy workload and interruption, and having to re-check doses frequently because of frequent change in patients' weight and surface area. Those related to motivation were: prescribers unquestioningly following recommendations and not communicating with other specialists because they over-trusted them or feared a negative reaction, and prescribers inability to complete a task because of other competing and preferable tasks at the same time. CONCLUSION: Employing COM-B helped in identifying causes of PEs from a new perspective. Future work could focus on mapping identified sources of behaviour and errors against appropriate intervention functions and policies in order to design more successful interventions.