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The dramatic growth of commodity index investment over the last decade has caused a heated debate regarding its impact on commodity prices among legislators, practitioners and academics. This paper focuses on the unique rolling activity of commodity index investors in the commodity futures markets and shows that the price impact due to this rolling activity is both statistically and economically significant. Two simple trading strategies, devised to exploit this market anomaly, yielded excess returns with positive skewness and annual Sharpe ratios as high as 4.4 in the period January 2000 to March 2010. The profitability of these trading strategies is decreasing in the amount of arbitrage capital employed in the futures markets and increasing in the size of index funds ’ investment relative to the total size of futures markets. Due to the price impact, index investors forwent on average 3.6% annual return, a 48 % higher Sharpe ratio of the return, and billions of dollars over thi