This article presents a model for the determination of crude oil futures price that focuses on the activities of arbitragers and speculators. Arbitragers, who exploit cost of carry deviations, rely (in part) on commodity convenience yield to identify mispriced futures contracts. However, an important impediment to this process is obtaining timely information regarding marginal convenience yield as crude oil inventory changes. This study employs an unconventional measure of convenience yield that allows for inferring inventory changes directly from spot and futures prices of the commodity. The model is tested for three and six month West Texas Intermediate futures contracts. The results show a decline in the effectiveness of arbitrage activity during the sharp rise and fall in crude oil prices in 2008. Over the same period, it is found that speculation played a greater role in the determination of crude oil prices.