An exposure programme was carried out to study the impact of traffic on building limestone soiling and decay. Roads with different traffic flows in Oxford were used in the study. After 1–3 years, it was found that busier roads experience faster soiling, and all sites showed extensive colonisation by bacteria, fungi, and phototrophs, deposition of particulate matter and calcite dissolution. In all cases, fungal colonisation is preferentially located within hollows on the stone surface. Fungi appear to be an important component of soiling as confirmed by SEM observations and isolation experiments. However, no clear pattern of microorganism colonisation was observed between sites, although the urban background site experienced higher colonisation of all organisms after three years. 15 refs.