This study explores whether mental representations based upon the Theory of Planned Behaviour (TPB) can predict engagement in rehabilitation after acquired brain injury (ABI). A scale was developed to measure: treatment outcome beliefs, perceived barriers, subjective norm, and control cognitions. Other adjustment factors that are often seen as important in predicting engagement, e.g., denial and anger, were also measured using the Motivation for Traumatic Brain Injury Questionnaire (MOT-Q). Clinicians also provided ratings of patient's engagement in rehabilitation which was used as the measure of actual behaviour. The scales were administered to 40 participants with ABI who were a mean of 14 months post-injury. The new scale showed good internal reliability, and regression analyses demonstrated that "treatment outcome beliefs" were the main aspects of TPB that predicted engagement. Control beliefs and adjustment factors added very little to the variance in engagement that was already explained by TPB. However, the amount of variance that was explained by TPB was small and it is suggested that other volitional factors need to be considered in order to predict engagement more accurately. This study thus constitutes a first exploration of mental representations about rehabilitation after ABI using TPB. It suggests a theoretical model that may help clinicians to formulate lack of engagement. It also suggests that addressing patients' beliefs about the benefits of undertaking rehabilitation may prove fruitful in increasing engagement.