This research is a psychotherapy process study, looking at the experience of clients in a session of Motivational Interviewing (MI), a therapeutic approach known to be effective in the treatment of substance use problems. Clients were interviewed using Interpersonal Process Recall (IPR) methodology, which is a structured interview method, utilising videotape of the MI session to aid client recall. Clients were asked to comment on the session, describing aspects that they experienced as important. The study then used Grounded Theory (GT) to analyse transcriptions of interviews. Through systematic coding of the data, categories emerged which were structured in a theoretical formulation of the client’s experiences of a session of MI. The results suggest that there are multiple intrapersonal and interpersonal factors that are perceived as being important when thinking about change in a session of MI. Clients reflected that therapist qualities, skills, and the experience of a therapeutic alliance enabled an interpersonal interaction that involved an open and honest discussion about alcohol use. Clients perceived that there were intrapersonal processes that were important in their experience of the session, these were increased awareness, self esteem, self efficacy, autonomy and activation of emotion. The social and clinical context of the session was also described as important. The results from the current study support the presence of multiple factors that have been identified in the literature as mechanisms of change in MI and also highlight the importance of common factors in therapy. Further work is necessary to look at the processes of change in MI and the interactional nature of the active ingredients.