Proposals concerning the future regime to govern deepsea mining put forward at the United Nations Conference on the Law of the Sea reflect a lack of empirically based criteria, particularly with reference to the impact of seabed mining on the metal markets and the redistribution of income implied. A thorough discussion of the issues at stake evidently calls for substantial quantitative work. The aim of the present paper is to contribute to the discussion of this topic as far as the market for manganese ore is affected. First, a dynamic commodity model for manganese is derived, specified and estimated. After establishing its recursive properties the model is then validated in historical simulation and stability analysis. Next, forecasts up to 1995 are made for supply, demand and the price of manganese. Finally, the model is simulated over alternative scenarios, in order to study the sensitivity of the manganese market to a supply shock from deepsea mining under different assumptions about the future economic environment.