In a closely controlled laboratory retorting system for oil shale, a relationship was developed by the Bureau of Mines between the temperature in the retort and the weight ration of ethylene to ethane in the gasses produced. Reciprocal temperature was shown to be direcly proportional to the log ([ethylene]/[ethanel]). From this relationship a number, called retorting but its magnitude depends on both temperature and residence time for the reactants in the heated zone in the oil-shale retort. The slopes of the lines relating reciprocal temperature and the logarithm of the ethylene-ethane ratons are nearly constat for each of the retorting procecsses for which data have been presented. The retorting index should be a valuable toolto control the operation of oil-shale retors, either aboveground or in situ. If an optimum operating condition can be established, the maintenance of the optimum or the departure from it should be easily monitored by use of the index.