This study examines the exercise of discretion by investor-state arbitral tribunals in treaty interpretation. It builds upon the premise that it is an illusion to pursue a single right answer to a given legal problem when the human experience is marked by an incomplete and fragmented perception of reality. Adjudication and legal interpretation, therefore, are not understood in this study as a matter of discovery or illumination, but as a matter of choice and persuasion over choices made. Even still, the exercise of hermeneutical discretion does not amount to a license for arbitrary behavior. There may not be one single correct reading of a treaty, but certainly not all interpretations of a treaty are possible. Notably, the semantic horizon of treaty interpreters is a finite one, as the methodology established by the Vienna Convention on the Law of Treaties (“VCLT”) constrains the hermeneutical choices open to interpreters. Article 31 lays out a general rule of treaty interpretation, while Article 32 provides supplementary means of interpretation that can be used either to validate an outcome reached through Article 31 or, in exceptional circumstances, to autonomously determine the meaning of a treaty. Within this framework, this study examines interpretative practices of investment-treaty arbitral tribunals in light of the interpretive regime established by the VCLT, exploring their conformity or non-conformity to that regime.