The hypothesis of this paper is that crude oil, like any other unfinished commodity, is valued for the products derived from it; the purpose is to offer an empirical explanation for changes in the crude price charged by the members of OPEC. The model results show that the market-clearing prices reported to prevail for petroleum products on the principal petroleum spot market at Rotterdam are the primary determinants of changes in official crude prices. A systematic relationship between offical and spot prices is argued to have prevailed since 1974. An appendix clarifies five types of data required for the model. 13 references, 4 tables.