The gas-phase hydrolysis of thionyl chloride (SOCl2) has been investigated at 297 and 309 K in a mixing chamber using FTIR spectroscopy. Reagent concentrations of similar to80 ppmv SOCl2 at relative humidities (RH) of 1-63% were studied at 309 K, while humidities of 1-85% were used in the 297 K studies, all at a total pressure of similar to1 atm in synthetic air. In each experiment, an aliquot of SOCl2(g) was rapidly introduced into a chamber at fixed RH, quickly reaching a maximum and then decreasing exponentially, as monitored by time-resolved infrared spectroscopy. The only observed reaction products were HCl(g) and SO2(g), which were formed in a molar ratio of similar to2:1. The SOCl2 decay data are consistent with, but do not prove, the reaction being a gas-phase hydrolysis that is first order in both SOCl2 and H2O, with the RH-dependent decay data indirectly suggesting the first-order dependence on H2O(g). The 297 K rate constant was measured as (6.3 +/- 3.5) x 10(-21) cm(3)/molecule-s. Confirming previous results, SOCl2 absorbed only at lambda < 290 nm, which suggests hydrolysis as the main loss mechanism in the troposphere at typical temperatures and humidities.