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The kinetics of the i -C 4 H 5 (buta-1,3-dien-2-yl) radical reaction with molecular oxygen has been measured over a wide temperature range (275-852 K) at low pressures (0.8-3 Torr) in direct, time-resolved experiments. The measurements were performed using a laminar flow reactor coupled to photoionization mass spectrometer (PIMS), and laser photolysis of either chloroprene (2-chlorobuta-1,3-diene) or isoprene was used to produce the resonantly stabilized i -C 4 H 5 radical. Under the experimental conditions, the measured bimolecular rate coefficient of i -C 4 H 5 + O 2 reaction is independent of bath gas density and exhibits weak, negative temperature dependency, and can be described by the expression k 3 = (1.45 +/- 0.05) & times; 10 & minus;12 & times; ( T /298 K) & minus;(0.13 +/- 0.05) cm 3 s & minus;1 . The measured bimolecular rate coefficient is surprisingly fast for a resonantly stabilized radical. Under combustion conditions, the reactions of i -C 4 H 5 radical with ethylene and acetylene are believed to play an important role in forming the first aromatic ring. However, the current measurements show that i C 4 H 5 + O 2 reaction is significantly faster under combustion conditions than previous estimations suggest and, consequently, inhibits the soot forming propensity of i -C 4 H 5 radicals. The bimolecular rate coefficient estimates used for the i -C 4 H 5 + O 2 reaction in recent combustion simulations show significant variation and are up to two orders of magnitude slower than the current, measured value. All estimates, in contrast to our measurements, predict a positive temperature dependency. The observed products for the i -C 4 H 5 + O 2 reaction were formaldehyde and ketene. This is in agreement with the one theoretical study available for i C 4 H 5 + O 2 reaction, which predicts the main bimolecular product channels to be H 2 CO + C 2 H 3 + CO and H 2 CCO + CH 2 CHO. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.