Researchers and practitioners alike see great value in understanding the implementation, adoption, and use of technology, and acknowledge the need to better understand post-adoption behaviour. Among theories that explain and predict human behaviour, attribution theory is recognised for its extensive investigation of behaviour's antecedents and consequences. This paper provides an overview of the theory, suggests a practical method for using it in IS contexts, and shows evidence that this method provides meaningful results. In order to address the complexities encountered in field-work, this paper argues that system-usage can be treated as an interpersonal relationship between the user and the system. This perspective allows us to draw on extensive knowledge gained in the field of interpersonal relationships research, in particular a relationship diagnostics method that uses interview data, followed by an analysis of the attributions mentioned in the interviews. The paper provides evidence from a study that successfully used attribution theory in this way to investigate a non-interpersonal relationship - an employee-organisation relationship. The paper concludes with suggestions for future research in IS based on this method.