This paper investigates the impact of ethanol on the oxidation of iso-octane at low and intermediate temperatures. Flow reactor experiments are conducted on binary mixtures ranging from neat iso-octane to neat ethanol at temperatures of 650 K and 875 K, a pressure of 10 bar and an equivalence ratio of 0.058, with the intermediate species and gas temperatures measured along the reactor. At 650 K, where neat iso-octane is near the peak of its low temperature chain branching, the addition of small amounts of ethanol dramatically suppresses iso-octane's reactivity and formation of intermediate species. Kinetic simulation reveals that this inhibition effect is a result of ethanol competing for OH radicals and producing an imbalance in OH formation, without affecting the iso-octane oxidation mechanism directly or altering its reaction flux distribution. By consuming more OH than it can produce, oxidation of ethanol progressively depletes OH in the radical pool, even at a low blending level, and leads to a strong reduction in the overall reactivity. In contrast, at 875 K, iso-octane and ethanol show comparable reactivities, and the formation of intermediate species correlates roughly linearly with the ethanol content, suggesting negligible interaction between the two fuels at intermediate temperatures. Overall, these results help to explain ethanol's inhibition on the knock propensity of iso-octane in spark ignition engines, as evidenced by the synergistic blending of the two fuels in octane numbers [1]. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.