A step-scan Fourier-transform spectrometer coupled with a 6.4 m multipass absorption cell was employed to detect time-resolved infrared absorption spectra of the reaction intermediate CH3SO2 radical, produced upon irradiation of a flowing gaseous mixture of CH3I and SO 2 in CO2 at 248 nm. Two transient bands with origins at 1280 and 1076 cm-1 were observed and are assigned to the SO 2-antisymmetric and SO2-symmetric stretching modes of CH3SO2, respectively. Calculations with density-functional theory (B3LYP/aug-cc-pVTZ and B3P86/aug-cc-pVTZ) predicted the geometry, vibrational, and rotational parameters of CH3SO2 and CH3OSO. Based on predicted rotational parameters, the simulated absorption band of the SO2-antisymmetric stretching mode that is dominated by the b-type rotational structure agrees satisfactorily with experimental results. In addition, a band near 1159 cm-1 observed at a later period is tentatively attributed to CH3SO2I. The reaction kinetics of CH3 + SO2→CH3SO 2 and CH3SO2+I→CH3SO 2I based on the rise and decay of absorption bands of CH 3SO2 and CH3SO2I agree satisfactorily with previous reports.