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An adaptation of the procedure originally developed by Twu and Coon for blend octane prediction is described. The technique is based on a graded index describing an aspect of the negative temperature coefficient (NTC) autoignition behaviour of a fuel. It is further postulated that the fuel's NTC behaviour can be linked to the transition state activation energy barriers involved in the first internal hydrogen abstraction by the alkylperoxy free radical. Density-functional theory (DFT) calculations were employed to assess this hypothesis and the results were able to explain the difference between the ignition behaviour of a number of selected fuel components. The calculated NTC assignments, which were directionally consistent with the DFT results, were used successfully to determine the blend octane rating.