This paper looks at the evolution of the terms of trade between commodities and manufactures in the twentieth century. A statistical analysis of the relative price series for 24 commodities and eight indices reveals a significant deterioration in their barter terms of trade over the years 1900 to 2000. This decline was not continuous, however, nor was it distributed evenly among individual products. As is shown here, 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 the aggregate indices of real raw material prices.