This Article explores the international law applicable to the exploitation of hydrocarbon resources that straddle the boundaries between States (transboundary fields) and its applicability to the U.S. and Mexico maritime boundary in the Gulf of Mexico. After a detailed examination of the different sources of international law including treaties, customary norms, judicial decisions, and bilateral practice, the Article concludes that the United States and Mexico have deviated in some regards from the standard international legal practices that other States have adopted to exploit transboundary hydrocarbon resources. The two most notable deviations are in allowing either nation to unilaterally exploit the shared resource, if no unitization agreement can be achieved, and the absence of an effective third party dispute settlement mechanism under some circumstances. Concretely, the Article analyzes how the latest instrument signed by these two nations for the exploitation of these resources, the 2012 Agreement on the Exploitation of Transboundary Hydrocarbon Resources, modifies international practice in several aspects and has the potential of complicating the efficient exploitation of the resources for the benefit of both nations. In reaching its conclusion the Article reviews the ratification processes of the agreement, the legal implications on the way both States perceive the use of these resources, and the effects that the 2014 reform in the energy sector in Mexico has on the binational treaty regime.