Infrared spectra are reported of Pd/SiO2 and Pd-Cu/SiO2 exposed at 295 K to either CO followed by SO2 or SOz followed by CO. Chemisorptive interaction of SO2 with bridging CO sites on Pd {100} faces created cationic Pd sites which adsorbed CO linearly. Multibonding CO sites on Pd {111} faces and linear CO sites were poisoned by weak non-dissociative SO2 adsorption, and could be regenerated by evacuation. Pd/SiO2 was more resistant than Cu/SiO2 to SO2 poisoning but for Pd-Cu/SiO2 the poisoning of Pd was promoted by the presence of Cu.