C H3 OO radicals were produced upon irradiation of a flowing mixture of C H3 I and O2 with a KrF excimer laser at 248 nm. A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to record temporally resolved IR absorption spectra of reaction intermediates. Transient absorption bands with origins at 3033, 2954, 1453, 1408, 1183, 1117, 3020, and 1441 cm-1 are assigned to 1 - 6, 9, and 10 modes of C H3 OO, respectively, close to wavenumbers reported for C H3 OO isolated in solid Ar. Calculations with density-functional theory (B3LYP/aug-cc-pVTZ) predicted the geometry and the vibrational wavenumbers of C H3 OO; the vibrational wavenumbers and relative IR intensities of C H3 OO agree satisfactorily with these observed features. The rotational contours of IR spectra of C H3 OO, simulated based on ratios of predicted rotational parameters for the upper and lower states and on experimental rotational parameters of the ground state, agree satisfactorily with experimental results; the mixing ratios of a -, b -, and c -types of rotational structures were evaluated based on the direction of dipole derivatives predicted quantum chemically. A feature at 995 cm-1, ascribed to C H3 OOI from a secondary reaction of C H3 OO with I, was also observed.