This work describes in situ infrared reflection absorption spectroscopy (IRAS) studies of the initial interaction of platinum and palladium to humidified air into which SO2 has been introduced. The formation of surface species could be followed in situ from 9 min to 20 h of exposure. Supported by X-ray photoelectron spectroscopy and glancing incidence X-ray diffraction analysis after exposure, the IRAS bands from surface species formed in the aqueous adlayer on both metals has been assigned to sulfuric acid, H2SO4, with no evidence of chemical reaction with the metal. When changing from humid to dry exposure conditions, the aqueous adlayer with sulfuric acid transforms into one or more phases of hydrated crystalline sulfuric acid, H2SO4 . nH(2)O. On platinum there is observed an initially fast formation rate of sulfuric acid followed by a slower formation rate with prolonged exposure. The initial formation rate on palladium, however, is much slower than on platinum. After about 7 h of exposure, a sudden but well-reproduced increase occurs, whereafter the formation rate levels off.