Physical activity levels in young adults are low. Research supports the use of the Transtheoretical Model of behaviour change (TM) in designing physical activity interventions. This study used a pre–post randomized control design to investigate the effectiveness of a self-instructional intervention for helping sedentary young adults to initiate physical activity. Post-intervention, significantly more of the experimental group (80%), in comparison to the control group (68%), improved their exercise stage of change (SOC) from baseline (P < 0.05). Discriminant analyses revealed that discrimination between stage improvement/non-improvement was possible using the processes of change data. Stage improvers scored significantly higher on all of the behavioral and four out of five of the cognitive processes of change. For stage improvers, the processes of self-re-evaluation and self-liberation were most frequently used, whilst social liberation was used significantly more by the experimental than the control group. This inexpensive, self-instructional intervention, based on the TM and the `active living message', is an effective method of assisting sedentary young adults to progress through the exercise SOC.