Starting from the recent discovery of chlorine ions in Io's plasma torus, we searched for evidence of Cl-bearing species at the surface of the satellite. We have identified Cl 2 SO 2 , with possible contribution by ClSO 2 , as candidates for the absorber(s) of the 3.92 mm band locally present in NIMS/Galileo spectra of the reddish deposits south of Marduk's volcanic center. Low-temperature laboratory measurements of the infrared spectra of several Cl and S-bearing molecules in the solid state, coupled with radiative transfer modeling, first allowed us to select four candidate molecules. Their abundance and stability at Io's surface have been tested through formation, condensation, and destruction scenarios using volcanic and atmospheric models completed with chemical and thermodynamical data. In particular, the sublimation rates of solid Cl 2 SO 2 and SO 2 have been measured to study the selective condensation of these species. Cl 2 SO 2 diluted at $1% in a millimeter thick layer of solid SO 2 is the favorite candidate for the 3.92 mm band. We strongly favor a formation process of this molecule by heterogeneous reaction of Cl atoms on SO 2 ice condensing on plume particles or at Io's surface. The high Cl 2 SO 2 abundance observed implies that a Cl-rich volcanic eruption ([Cl À (Na + K)]/S > 0.015) occurred at Marduk. ClSO 2 is a potential additional contributor to the band. Pure H 2 S is safely discarded as it is extremely unstable at Io's surface but an upper limit of 0.01% is derived for H 2 S diluted in SO 2. Finally, chemical constraints allow us to firmly exclude H 2 S 2. We also suggest that Cl 2 S may be an alternative explanation for the reddish coloration of some volcanic deposits.< Réduire