The lichen composition on wayside Quercus robur in the Netherlands was related tobark properties (pH, EC, NH4+, SO42-, NO3-)and levels of air pollution (SO2 and NH3). The pH of the bark and the susceptibility to toxic substances appear to be the two major primary factorsaffecting epiphytic lichen composition. These factors have independent effects on the lichen composition.Most of the so-called nitrophytic species appear to have a low sensitivity to toxiceffects of SO2; their only requirement being a high bark pH. An increased bark pH appears to be the primary cause of the enormous increase in nitrophytic species and the disappearance of acidophytic species over the last decade in the Netherlands. Measurements of ambient NH3 concentrations in air show that there is a nearly linear relationship between the NH3concentration andthe abundance of nitrophytes on Quercus. The abundance of nitrophytes was not correlated with SO2 concentrations. Most of the acidophytic species appear very sensitive to NH3 since in areas with concentrations of 35 µg m-3 or more, all acidophytic species have disappeared. Current methods using species diversity to estimate or monitor SO2 air pollution need some modification, otherwise the air quality may be erroneously considered to be relatively good in areas with high NH3 levels.