This article considers whether an international investment agreement (IIA) between two states parties can be interpreted in the light of a ‘third-party IIA’ (defined as a party’s IIA with a third state, a party’s model IIA or an IIA between other states parties). A significant number of tribunals have been willing to interpret the IIA before them with reference to third-party IIAs, drawing inferences from differences or similarities in their texts. However, the use of third-party IIAs in this manner often reflects an erroneous application of the customary rules of treaty interpretation set out in Articles 31–33 of the Vienna Convention on the Law of Treaties (VCLT). These conclusions have significant implications for international investment law and state practice. If investment tribunals continue to look to third-party IIAs beyond the parameters of the VCLT, beyond consent of the disputing parties and beyond the common intention of treaty parties, contemporary developments in treaty drafting may have unintended or even perverse consequences.