A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to detect temporally resolved infrared absorption spectra of CH3OSO produced upon irradiation of a flowing gaseous mixture of CH3OS(O)Cl in N2 or CO2 at 248 nm. Two intense transient features with origins near 1152 and 994 cm-1 are assigned to syn-CH3OSO; the former is attributed to overlapping bands at 1154 ± 3 and 1151 ± 3 cm-1, assigned to the SO stretching mixed with CH3 rocking (8) and the SO stretching mixed with CH3 wagging (9) modes, respectively, and the latter to the C-O stretching (10) mode at 994 ± 6 cm-1. Two weak bands at 2991 ± 6 and 2956 ± 3 cm -1 are assigned as the CH3 antisymmetric stretching ( 2) and symmetric stretching (3) modes, respectively. Observed vibrational transition wavenumbers agree satisfactorily with those predicted with quantum-chemical calculations at level B3P86aug-cc-pVTZ. Based on rotational parameters predicted at that level, the simulated rotational contours of these bands agree satisfactorily with experimental results. The simulation indicates that the SO stretching mode of anti-CH3OSO near 1164 cm-1 likely makes a small contribution to the observed band near 1152 cm-1. A simple kinetic model of self-reaction is employed to account for the decay of CH3OSO and yields a second-order rate coefficient k (4 ± 2)10-10 cm3 molecule -1 s-1.