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Net heat release rates and knock characteristics were derived from in-cylinder pressures for different fuels in a single-cylinder engine; the effect of an ashless antiknock, N-methyl aniline (NMA) was also studied. The maximum net heat release rate (MHRR) resulting from the final high-temperature chemistry determines the knock intensity. Paraffinic fuels have similar knock intensities at comparable knock occurrence frequencies. Aromatic fuels have significantly lower MHRRs and give much lower mean knock intensities for a given knock occurrence frequency compared to paraffinic fuels. Adding NMA to a paraffinic fuel increases the spark advance required to get a chosen frequency of knock occurrence as it increases the octane number of the fuel but has little effect on MHRR and hence knock intensity.