This experimental study was conducted in a motored research engine to investigate the effect of blending methanol and ethanol on hydrocarbon oxidation and autoignition. An 87 octane mixture of primary reference fuels, 87 PRF, was blended with small percentages of the alcohols to yield a constant gravimetric oxygen percentage in the fuel. The stoichiometric fuel mixtures and neat methanol and ethanol were tested in a modified single-cylinder engine at a compression ratio of 8.2. Supercharging and heating of the intake charge were used to control reactivity. The inlet gas temperature was increased from 325 K to the point of autoignition or the maximum achievable temperature of 500 K. 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 molar fraction profiles of in-cylinder stable species as a function of crank angle degree for all of the fuels at a constant inlet condition. Results indicated that both alcohols reduced the overall levels of CO formation and increased the inlet temperatures required to initiate oxidation and generate autoignition of 87 PRF. On a constant gravimetric oxygen basis, ethanol reduced reactivity more than methanol. The underlying chemical mechanism for inhibition of autoignition is discussed based on in-cylinder species data analysis. The difference in the behaviors of these alcohols is discussed in terms of their molecular structure and oxidation chemistry.