This paper presents a comparison of the autoignition tendencies for straight chain hydrocarbons and mixtures of primary reference fuels in a motored engine. Minimum initial gas temperatures required to produce autoignition were measured as a function of fuel type, engine speed, and inlet manifold pressure. In-cylinder gas pressure vs. crank angle and exhaust gas carbon monoxide concentrations were also monitored during these experiments as indicators of chemical activity. Overall, the autoignition behavior of n-pentane, n-hexane, and their equivalent octane number primary reference fuel blends was found to be dissimilar in (i) the inlet temperatures at which autoignition occurred; (ii) the amount of CO formed prior to autoignition; and (iii) the effect of engine speed on minimum inlet autoignition temperatures. Possible causes for this behavior are discussed in the paper in terms of the Negative Temperature Coefficient behavior of large hydrocarbons.