This article is about state responsibility and its unique interaction with environmental law. While remedies in the main are reparative in nature, the ‘guarantees of non-repetition’ are qualitatively distinct, intended to prevent recurrence of a breach and, as such, this remedy brings added value to environmental law. Utilizing the Montara oil spill as a conceptual testing ground, this article argues that the future-oriented guarantees of non-repetition create an untapped opportunity for an injured state. Benefiting from the leverage attached to receiving guarantees of non-repetition, an injured state may evoke the International Law Commission’s Articles on Prevention of Transboundary Harm to negotiate future prevention and, where it sees fit, to seek to institutionalize future oversight by various joint-monitoring mechanisms, going so far as to call for a bilateral intergovernmental organization.