A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of benzoyl radical, C 6H 5CO, with O 2. C 6H 5CO was produced either from photolysis of acetophenone, C 6H 5C(O)CH 3, at 248 nm, or from photolysis of a mixture of benzaldehyde, C 6H 5CHO, and Cl 2 at 355 nm. Two intense bands near 1830 and 1226cm -1 are assigned to the C=O stretching ( 6) and the C-C stretching mixed with C-H deformation ( 13) modes, and two weaker bands near 1187 and 1108cm -1 are assigned to the 14 (C-H deformation) and 16 (O-O stretching C-H deformation) modes of C 6H 5C(O)OO, the benzoylperoxy radical. These observed vibrational wave numbers and relative infrared intensities agree with those reported for syn-C 6H 5C(O)OO isolated in solid Ar and values predicted for syn-C 6H 5C(O)OO with the B3LYPcc-pVTZ method. The simulated rotational contours of the two intense bands based on rotational parameters predicted with the B3LYPcc-pVTZ method fit satisfactorily with experimental results.Additional products BrCO, C(6)H(5)C(O)Br, and C(6)H(5)C(O)C(6)H(5) were identified in the C(6)H(5)Br/CO/N(2) experiments; the kinetics involving C(6)H(5)CO and C(6)H(5)C(O)Br are discussed.