In general, the rate of heat release during combustion in a spark ignition engine, can have two components: one due to normal burning in a propagating flame, and another due to autoignition in the end gas. It has been possible to separate these two components by analysing the pressure trace of a single cylinder engine. From this, the volumetric autoignition heat release rate can be inferred and studied in some detail. To approximate this rate in an Arrhenius form presents difficulties, in so far as it is not possible to measure the temperature at the instant of maximum heat release rate, at the onset of knock. However, it was possible to measure end gas temperatures by the CARS technique prior to autoignition and then to estimate the temperature at the onset of autoignition by extrapolation. Estimation of the temperature at the instant of maximum heat release rate has enabled kinetic parameters to be assigned in an Arrhenius expression for this rate over a range of temperatures. These parameters have been measured for fuels of different octane number, and for an aromatic and paraffinic fuel blend of identical MON at different mixture strengths. From these parameters and an earlier analysis of the coupling of pressure waves generated during autoignition with chemical reaction, the likelihood of a particular fuel blend giving rise to a particular mode of autoignition can be assessed. The most damaging mode is a developing detonation, which occurs at high values of the autoignition volumetric heal release rate. Such rates are higher for paraffinic than for aromatic fuels under the same conditions of temperature and pressure. As a result, a developing detonation is more likely for a paraffinic than for an aromatic fuel blend. For the paraffinic blends, the lower are both the octane number and the air/fuel ratio, the higher is the volumetric heal release rate, and hence also the knock intensity.