A new species, ClSO2, is produced and identified with infrared (IR) absorption spectra when an argon or krypton matrix containing Cl2 and SO2 is irradiated with laser emission at 355 nm. Lines at 1311.0, 1309.6, 1099.8, 1098.2, 497.7, and 455.8 cm-1 (or 1309.5, 1098.5, 497.0, and 454.2 cm-1) are assigned to ClSO2 isolated in solid Ar (or Kr). Assignments of IR absorption lines are based on results of 34S and 18O isotopic substitution (in solid Kr) and theoretical calculations. Theoretical calculations using density-functional theories (B3LYP and B3P86 with an aug-cc-pVTZ basis set) were performed to predict the geometry, energy, vibrational frequencies, and infrared intensities of possible isomers of ClSO2: (pyramidal) ClSO2, cis-ClOSO, (nonplanar) ClOSO, and cis-ClSOO. Results predicted for pyramidal ClSO2 agree with observed experimental data. This is the first identification of ClSO2, which is presumably an important intermediate during photolysis of Cl2SO2 and in the reaction of Cl with SO2, especially at low temperatures. In addition to ClSO2, irradiation of the Cl2/SO2/Ar matrix sample with laser light at 308 nm produces Cl2SO2. Possible mechanisms of formation are discussed.