Cognitive behaviour therapy (CBT) is the most empirically supported treatment for bulimia nervosa (BN) and related disorders, yet approximately 1/3 of completers do not remit, there is a substantial treatment attrition rate, and 1/3 of remitted patients will relapse within the first 2 years. Improving CBT is an important research prerogative. Motivational interviewing (MI) is an augmentative intervention that has been investigated to improve CBT, but reviews indicate that it is not efficacious with eating disorders. The failure of MI with eating disorders has led to consideration that focusing on early behaviour change might be a more productive therapeutic strategy. Rapid response to treatment for eating disorders has been reliably identified as a prognostic indicator in eating disorders. For BN and similar disorders, rapid response is the rapid reduction of binge eating, vomiting, and dietary restriction during the first few weeks of treatment. Numerous studies have indicated that patients who rapidly respond to treatment are significantly more likely to be remitted at post treatment and significantly less likely to relapse, compared to those who respond more slowly. No preexisting clinical, demographic, personality or other factors have clearly emerged to account for this effect. Given that rapid response has clear prognostic importance, and given that research has failed to identify mechanisms driving this finding, this study seeks to determine whether rapid response can be facilitated clinically using a targeted intervention designed to provide patients with specific behavioural skills to decrease their bulimic symptoms rapidly. Improving remission and relapse rates is a high priority in the eating disorders research field. the investigators already know that cognitive and behavioural strategies have efficacy for eating disorders, but existing treatments need improvement. Rapid response is an area that has been frequently described and has clear prognostic importance and no clear mechanism accounting for why some patients rapidly respond, suggesting that perhaps rapid response could be facilitated if patients are provided with the skills, mindset, and support to do so. However, no study to date has sought to answer this question and determine whether rapid response can be facilitated in order to improve patient prognoses. Thus, this study seeks to examine whether rapid response to day hospital treatment can be facilitated clinically using an augmentative CBT based intervention targeting early symptom change. The CBT intervention will be compared to a matched-intensity motivational interviewing (MI) intervention. The rationale for using MI is because MI is frequently used to augment standard treatments, it provides an active treatment comparison, and because the rationale for the present study emerged partly from research aimed at understanding some of the limitations of MI in treating eating disorders. Thus, MI provides a theoretically-driven comparison group. It is predicted that individuals who receive CBT (versus MI) will be more likely to be classified as rapid responders, and will have fewer bulimic symptoms at post-day hospital and 6-month follow-up. As well, it is hypothesized that changes in self-efficacy, motivation, or hopefulness may help to account for these findings.