A numerical investigation was conducted to study the effects of di-tert-butyl peroxide (DTBP) additive on the reactivity of methanol and ethanol fuels. First, a reduced primary reference fuel (PRF)-methanol-ethanol-DTBP mechanism was proposed to simulate the homogeneous charge compression ignition (HCCI) combustion processes of PRE and alcohol-DTBP fuel mixtures. By linking through the combustion phasing of HCCI operation with the PRF fuels, effective PRF number maps were generated for the alcohol-DTBP fuels. The agreement between experimental and simulation results was reasonably good. Both the experiments and simulations showed that DTBP enhances the fuel reactivity of the alcohols and that the rate of reactivity enhancement decreases with increasing DTBP percentage. The reasons for the enhancement of reactivity by DTBP addition to both methanol and ethanol fuels were then explored kinetically. It was found that both thermal and chemical effects contribute to the reactivity enhancement, and this can be attributed to the heat released in the DTBP decomposition process, the reactive radicals generated through the CH3 --> CH3O2 --> CH3O2H --> OH pathway, and the reaction pathway of fuel + CH3O2 --> CH3O2H --> OH. The major reason for the different response of DTBP between methanol and ethanol was found to be the higher DTBP content in methanol-DTBP mixtures for the same operating conditions, and this was further confirmed by the fact that the effects of DTBP addition on methanol and ethanol reactivity were quite similar if the same absolute DTBP mass was added to these two alcohols.