Olefins comprise one of the major groups of hydrocarbons in shale-oil and cracked-petroleum naphtha's. A quantitative determination of this group of compounds may be made with reasonable accuracy by either the nitrogen tetroxide absorption or silica-gel absorption method. Calculation of olefins from bromine number, this method used extensively in the petroleum industry, gives high results. The absolute variation of shale-oil naphtha's may be as much as 35 percent. Consequently, bromine number seldom provides a satisfactory estimate of olefins when the content of these compounds is high. Sulfur and nitrogen compounds cause large errors in olefin content as determined by bromine number. Also substitution reactions and unequal distribution of the olefins throughout the boiling range probably play an important part. An accurate determination of olefins is extremely important in processing studies of materials such as shale oil naphtha that contain a high concentration of these compounds. In the past, the index of reliability of methods for determining olefins usually has been based on the titration of pure olefins or on the analysis of relatively simple synthetic mixtures. Little information is available upon the reliability of results that may be obtained when methods so tested are applied to complex samples. This article presents a direct comparison of results obtained by applying three olefin determination methods-bromine number, nitrogen tetroxide absorption and silica-gel adsorption-to a number of shale oil and petroleum naphthas.