Through the BEPS Project, the OECD is currently attempting to address the international tax system’s ongoing problems of base erosion and profit shifting. It aims to do so by introducing additional coordinated measures, while also rethinking and improving existing measures. The purpose of this article is to expose an issue that has not been addressed by the BEPS Project, though it is relevant to the Project’s objectives. This issue is the roles that contract interpretation law and private international law necessarily have in the task of delineating the actual controlled transaction when conducting the transfer pricing arm’s length comparability analysis. The article also explains some of the implications of these roles, and proposes measures for reform to address the potential risks involved.