The International Maritime Organization (IMO) is the United Nations specialized agency responsible for safe, secure and efficient shipping and the prevention of ship-sourced pollution. Responding to increasing environmental pressures to tackle ship-sourced GHG emissions, IMO adopted the 2023 IMO Strategy on Reduction of GHG Emissions from Ships (2023 Strategy) which significantly accelerated ambitions to decarbonize international shipping. To meet the new targets, the shipping industry and port States have rapidly increased research into alternative shipping fuels which produce low- or zero-GHG emissions. Ammonia and methanol have emerged as two of the most promising options. This article addresses the physical characteristics of fuels, IMO’s internal regulations and policy options. It examines how the move to use methanol and ammonia as alternative fuels for shipping could meet the IMO’s ambitions under its 2023 GHG Strategy. Further, it argues that while the efforts to demonstrate their feasibility as marine fuels are essential, the impact of methanol and ammonia fuels on human safety and on the marine environment will have to be given greater emphasis by IMO going forward. It argues that a knowledge gap currently exists on the impact of ammonia and methanol on the marine environment and on human safety. Consequently, it argues that the IMO should develop a comprehensive strategy and offers policy recommendations which incorporate the impact of the new fuels on human safety and protection of the marine environment.