The second half of the twentieth century saw major improvements in the legal regime for fisheries management. This notwithstanding, the deep seas remain largely unregulated under international law, until recently only being covered by the general environmental and management provisions found in UNCLOS. In light of this regulatory gap, this article evaluates the potential law-making effects, if any, of the FAO Deep-Sea Fisheries Guidelines, a voluntary instrument designed to provide States with a regulatory framework for the management of their deep-sea fisheries. It considers how the Guidelines may inform, interpret and influence the content of the general high-sea obligations in UNCLOS. Despite the vagueness and generality of those provisions, some indication of their substantive content has been given in recent decisions, particularly the South China Sea Arbitration. By assessing their compatibility, and their level of acceptance by the international community, this article argues that the FAO Deep-Sea Guidelines are beginning to have a law-making effect by providing an authoritative interpretation of the general high-sea obligations found in UNCLOS relating to deep-sea fisheries.