Abstract The multichannel reaction of excited singlet oxygen atom with ethanol, O(1D) + C2H5OH (1), was studied in a photolysis flow reactor coupled with mid-infrared Faraday rotation spectroscopy (FRS) and UV-IR direct absorption spectroscopy (DAS) at 297 K with reactor pressures of 60, 120, and 150 Torr (bath He). The excited singlet oxygen atom was generated through the photolysis of O3 at 266 nm. The photon flux and O(1D) concentrations were determined by in situ actinometry based on O3 depletion. Temporal profiles of OH and H2O were monitored via DAS signals at ca. 3568.62 and 3568.29 cm−1, while temporal profiles of HO2 were measured via FRS signals at ca. 1396.90 cm−1. The branching ratios of the target reaction (1) were determined by fitting temporal profiles to simulations from an in-house reaction mechanism. Two major reaction channels were identified as CH3CHOH + OH and CH3O + CH2OH, and their branching ratios were determined as 0.46 ± 0.12 and 0.42 ± 0.11, respectively. A specific HO2 + RO2 reaction between HO2 and O2CH2CH2OH (β-RO2) at the low-temperature range is estimated in this work as HO2 + O2CH2CH2OH products with a rate constant of 7 × 10−12 cm3 molecule−1 s−1.