Autoignition characteristics of neat fuels consisting of ethanol, n-butanol, n-heptane (nC7), methyl hexanoate (mhx), and methyl 3-hexenoate (m3h) and their binary and ternary blends were studied in a motored Cooperative Fuels Research (CFR) engine under homogeneous charge compression ignition (HCCI) conditions. The equivalence ratio (ER) and intake temperature were fixed to 0.5 and 155 degrees C, respectively. Autoignition characteristics were studied through the evolution of CO/CO2 emissions and heat release rate profiles as the compression ratio (CR) was increased. The critical compression ratio (CCR), defined as the CR at which the onset of the autoignition occurs and identified here as the CR where CO2 increases dramatically, varied from 4.8 for neat nC7 to 14.1 for ethanol, which exhibited the lowest reactivity of all fuels. C7 methyl esters exhibited lower CCR than both alcohols, confirming the nonlinear relationship between autoignition trend and cetane number. Results showed that mxh exhibited cool-flame behavior, while m3h and the alcohols did not, and that both alcohols (ethanol to a greater extent than n-butanol) suppressed the low-temperature oxidation reactivity (LTOR) to a higher extent than the C7 methyl esters. Ethanol reduced the autoignition tendency of binary blends with nC7 by twice the extent of m3h and by triple the extent of mhx. The addition of ethanol of up to 20% induced a roughly linear effect on the autoignition characteristics in both binary and ternary blends, which confirms that the suppressing effect of ethanol is stronger than that of m3h addition. For a fixed alcohol content in the blend, the autoignition characteristics are highly nonlinear with the blend composition, and they are less sensitive to the least reactive components in the blend. Finally, it was found that, regardless of the fuel components, blends with similar autoignition characteristics exhibited similar thermal histories (low-temperature heat release, LTHR, rate peaks and onset of LTHR).