AbstractThe ‘official’ (OPEC) prices of crude oil before the collapse in the oil market in the mid‐1980s can be interpreted as contract prices and analysed on the basis of the theory of futures (or forward) markets. This paper uses the generalized method of moments estimation technique to test for efficiency in the relationship between the official prices and the ex‐post spot prices at the time of delivery. Efficiency is rejected for the sample period 1978–1985 as a whole, but evidence is found of improvements over time. Further, the GMM Wald and Hansen tests, although asymptotically equivalent, are shown to differ greatly when applied to a small sample of monthly oil p