Since 1989 parts of the Netherlands are investigated on lichen composition on the bark of the Common Oak (Quercus robur). The purpose of this survey is to assess the regional influence of ammonia-emissions. The study has revealed three new species for the Dutch lichen flora: Caloplaca herbidella. Lecidella flavosorediata and Protoparmelia oleagina. Physcia clementei has been found back since 1952. Anaptychia ciliaris and Ramalina lacera seem to be less rare than expected before.