Motivational interviewing (MI) is a client- centered, directive counselling method. This study evaluates the effectiveness of a two- day workshop in MI for forty social workers in changing self- reported practice over a three- month period, the levels of skills achieved, and factors associated with acquired skills, including the impact of post- workshop supervision. The focus of training was alcohol misuse, but participants were encouraged to explore the use of MI with other issues. A multi- method pre- and post- design was used, utilizing both quantitative and qualitative data and employing an embedded randomized controlled trial of the impact of supervision. The two- day workshop had a modest positive impact on evaluations of simulated practice, on some measures of attitudes to working with problem drinkers and in qualitative accounts of practice. Despite this, three months post- workshops, workers generally had not reached a skillful level of MI practice as measured in ratings of an interview with a simulated client. Offer of post- workshop supervision had little impact on skill, with take- up being low. There was a significant difference between participants in the two workshops, despite identical programmes and trainers. Qualitative data suggested that participants had found the training useful, and many reported a positive impact on their practice.