The research question addressed in this study is why Norway and Sweden embarked on a common support scheme for the promotion and use of renewable energy. This is an interesting question given the unique character of the joint Swedish-Norwegian green certificate scheme as the world’s first international market for trade in ‘green electricity’. The study analyzes the question through a ‘domain-of-application’ approach, using multiple theories to illuminate different stages of the process. Convergence theory has been employed as basis for investigating the emergence of similar preferences for the same policy tool in Sweden and Norway, complemented with perspectives of policy-learning. Domestic policy theory has been brought in to investigate specific national factors conditioning policy convergence and policy learning. International negotiation theory has been included for the purpose of investigating specifically the Swedish-Norwegian negotiation process which led to the final creation of a joint scheme. The main conclusions drawn from the study that the emergence of preferences for green certificates in the two countries was grounded in similar policy goals for renewable energy policy area as such, path-dependent policy tracks in relation to deregulated electricity markets and anticipated (but not realized) policy harmonization at the EU level, making green certificates more relevant for both countries.