In 1975, the local botanist H. Breuer published a list containing 67 species of bryophytes and 31 lichens from the Tomberg, a small hill near Rheinbach, district of Euskirchen, North-Rhine Westphalia. 20 years later, the area was re-investigated. 16 bryophyte species were discovered new, and 20 species listed by Breuer could not be confirmed. This points to strong dynamic processes operating in the bryophyte flora which can be attributed to changes in certain habitat conditions. Such marked changes in the flora can be interpreted as indicating unstable ecological conditions. The changes found at the Tomberg are much more pronounced than at comparable natural areas such as the Vosges, but much less pronounced than in areas which have undergone considerable ecological changes, like the North Sea island Juist. In the present study, a total of 49 lichen species were recorded for the Tomberg. A large proportion of those (23) found by Breuer were confirmed. 2 epiphytic species have most probably become extinct. The improvement in air quality, particularly the decrease in sulphur dioxide levels, appears to have enabled a number of epiphytic species to recolonize the area.