This chapter will focus on two issues discussed broadly by investment tribunals: rules on conflicts of treaty norms (Article 30 of the VCLT and corresponding CIL) and rules relating to subsequent agreements in relation to the interpretation of treaties (Article 31(3)(a) of the VCLT and corresponding CIL). With respect to the conflicts of treaty norms - a coherent line of tribunals decisions concerning relations between international investment agreements and EU treaties is introduced, which narrowly interpret the issue of ‘the same subject matter’. Regarding the rules on subsequent agreements several observations are made. First, the ease with which arbitral tribunals reject interpretations formulated by States through subsequent agreements is striking. Second, tribunals have set out detailed requirements for the formulation of a declaration, without explaining the source for these requirements. Third, there is a considerable difference between tribunals referring to joint interpretations of State Parties, as envisaged by the treaty as binding (such as with NAFTA), and the tribunal assessing such interpretations through the lens of the VCLT or corresponding customary rules.