Background It is essential to improve the motivation of incarcerated sexual offenders to engage in programmes that help lower reoffending rates. The Personal Aspirations and Concerns Inventory for Offenders (PACI‐O) is a semi‐structured interview that helps them to identify what they want to achieve or change in their life. It may improve treatment engagement. Aims The aims of this study were to test use of the PACI‐O as a motivator towards treatment engagement and to test the feasibility of a randomised controlled trial (RCT) of this with sex offenders. Methods Thirty‐seven convicted sex offenders who were due to participate in an Enhanced Thinking Skills programme were randomly assigned to the ‘experimental’ group, who completed the PACI‐O prior to programme entry, or a control group, who completed the programme but not the PACI‐O. All participants completed treatment engagement measures. Results Small positive changes were observed in indicators of treatment motivation and engagement, which were most marked in the small group of offence deniers. The RCT method proved feasible. Conclusions and implications for practice and research Although our sample was small, our preliminary finding indicates that the PACI‐O may provide a cost‐effective method of improving treatment engagement for a hard‐to‐reach group of offenders. Replication with a sample of just over 100 men should offer sufficient power for establishing this technique. Copyright © 2013 John Wiley & Sons, Ltd.