This study aims to provide a framework for assessing and modelling behavioural changes in a prison therapeutic community. A behavioural checklist was devised to monitor both positive and negative behaviours throughout the course of therapy, paying particular attention to offence-paralleling behaviours. Content analysis of therapy notes on 68 residents identified 35 variables. The study investigated the hypothesis that it is possible to model behavioural change over the course of therapy using an action systems framework. This framework has been used to classify different forms of anti-social behaviour and pro-social behaviour. The four modes of action system functioning were consistently identified during therapy using smallest space analysis. Paired sample t tests of residents at the beginning and end periods of therapy suggested that residents progress from anti-social to pro-social behaviours from within each mode. Overall, findings support the prison therapeutic community treatment model, and demonstrate the appropriateness of the action systems framework for understanding the nature and function of positive and negative therapeutic behaviours.