This article explores the tendency of investment tribunals to resort to teleological interpretation and to the protection and promotion of foreign investments as a standard goal of investment treaties. It further explores how this tendency relates to the rule of interpretation envisaged in Articles 31–33 of the Vienna Convention on the Law of Treaties considering that the convention rule requires that text, context, and purpose are to be equally assessed when searching for the meaning of a treaty provision. The article’s particular focus is on whether investment tribunals have begun to create specific rules for interpreting investment treaties that favor one of the elements of interpretation over the others, namely, the purpose of the treaty. This method of teleological interpretation is set against the general background of investment arbitration, the Vienna Convention on the Law of Treaties, and cases where investment tribunals’ reliance on the preamble of the applicable treaty was decisive for the final outcome of the case. These cases, which revolved around the goal to protect and promote foreign investments and teleological interpretation, lend support to the proposition that there has been a departure from the general rule. However, they equally show the unreliability of telos in a treaty: purposes may be different, even conflicting, and their clear meaning can easily escape interpreters. Despite a growing number of cases where the purpose of a treaty comes to the forefront of legal reasoning, the functional correlation between the purpose of a treaty and the legitimacy of a claim still remains unclear.