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This paper looks at the evolution of the terms of trade in commodities for the twentieth century. A statistical analysis of the relative price series for 24 commodities and of eight indices reveals a significant deterioration in their barter terms of trade over the course of the twentieth century. This decline was neither continuous, nor was it distributed evenly among individual products. However, the data shows that the far-reaching changes that the world economy underwent around 1920, and again around 1980, led to a stepwise deterioration which, over the long term, was reflected in a decline of nearly 1% per year in aggregate real prices for raw materials.