This paper presents results of studies investigating the effect of octane enhancing ethers on the reactivity of an 87 octane mixture of primary reference fuels, 87 PRF, in a motored engine. 87 PRF was blended with small percentages of MTBE, ETBE, TAME and DIPE based on a constant gravimetric oxygen percentage in the fuel. The experiments were conducted in a modified single-cylinder Wisconsin AENL engine at compression ratios of 5.2 and 8.2. Supercharging and heating of the intake charge were used to control reactivity. The inlet gas temperature was increased from 320 K, where no reactivity occurred, until either autoignition occurred or the maximum temperature of the facility was reached. Exhaust carbon monoxide levels and in-cylinder pressure histories were monitored in order to determine and quantify reactivity. A fast-acting sampling valve and GC analysis of the extracted samples were used to obtain the concentration profiles of in-cylinder stable species as a function of crank angle degree. Results indicated that while each ether reduced the overall levels of CO formation and increased the inlet temperature at which autoignition occurred, the magnitude of the effect varied. The underlying chemical mechanism for inhibition of autoignition is discussed based on species analysis using 87 PRF, 87 PRF/MTBE, and 87 PRF/DIPE mixtures. The difference in the behaviors of these ethers is discussed in terms of the molecular structure and oxidation chemistry.