This paper presents the results of an investigation cooperatively undertaken by Esso Research and Engineering Company and Ethyl Corporation to determine whether the hydrocarbon-type effect observed in road antiknock studies of gasolines is independent of other fuel properties over and above laboratory octane numbers.For this study, 51 finished gasolines were carefully blended from 57 base stock components to provide controlled levels of those major fuel properties which affect road performance. The controlled properties were Research octane number, sensitivity (RON minus MON), ratio of aromatics to olefins, tetraethyllead content, octane-number distribution in the fuel's boiling range, boiling-range location of the unsaturated hydrocarbons, and sulfur content. A unique feature of the blending scheme was the formulation of blend pairs, in which all but one of the major fuel properties were essentially equal.The gasolines were road rated in four recent-production cars, with the engine of one car modified to provide a compression ratio of 12-to-1. Ratings were made under part-throttle and maximum-knock operating conditions by both the Modified Uniontown and Modified Borderline techniques. The independent effect of each fuel property on road octane number was then evaluated both by regression analysis and by direct comparison of blend pairs.The results of this study show that, at the same Research and Motor octane numbers, the road superiority of an aromatic fuel over an olefinic fuel at moderate-to-high engine speeds varies with other compositional factors. For example, a leaded olefinic fuel can give road performance equal to that of a clear aromatic blend at speeds above 2000 rpm; this stems from the fact that leaded blends generally perform better than clear blends when all other properties are the same. The study also showed that the partial antiknock destruction of TEL by sulfur is such that the road performance of a leaded olefinic blend with low sulfur content can match that of a leaded aromatic blend with high sulfur content. At part throttle, the influence of these compositional factors becomes even more important than at full throttle. For example, at part throttle, the road performance of a leaded olefinic blend with low sulfur can exceed that of a leaded aromatic blend with high sulfur.