Increasing physical activity (PA) is a public health priority. There is preliminary support for application of Motivational Interviewing (MI) for physical activity (PA), web-applications for PA promotion, web-based MI for behavior change, and finally web-based MI for PA. Based on collective findings in these areas of inquiry, it is necessary to investigate the efficacy of MI as a method that lends readily to practical applications for PA promotion. The current study seeks to investigate whether web-based MI can be applied in an adult population to enhance PA participation using a physician referral scheme. Additionally, mechanisms of behavior change are considered in order to inform future research and intervention.