A synergistic effect on low-temperature autoignition reactivity of blending two high-octane compounds, i.e. cyclopentane and diisobutylene, was observed in engine experiments and ignition delay time measurements at 700-880 K and up to 25 bar with a rapid compression machine. Results show that the low-temperature ignition delay and RON of a cyclopentane-diisobutylene blend are inferior to those of its isolated blendstocks, therefore exhibiting an increased reactivity towards ignition at the tested conditions. Sampling and speciation of the reacting mixture during the ignition delay of pure compounds and the most reactive blend was conducted to support discussions focused on the kinetic perspective to this phenomena, which suggest that the separate chain initiation pathways of these potential highoctane blendstocks can interact with each other to help the radical pool growth at the beginning stage of the autoignition chemistry, and finally accelerate the propagation and branching pathways to promote the global reactivity. This result in significant two-stage ignition behavior for the blend, which is not observed at this temperature for both isolated blendstocks. These findings are especially meaningful for designing and blending suitable high-octane fuels for advanced combustion mode engines. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.