The pollution of the air with ammonia is in the Netherlands a big problem. Since 1989 lichens on oaktrees are used to map and monitor the effects of the pollution. About 40,000 trees are investigated. It appears possible to map the effects of ammonia without interference of sulphurdioxide. The NIW (“Nitrophilous Indication”) is calculated with the frequency and quantity of about twenty nitrophytic species in samples of ten trees. The results show a big resemblance with maps of ammonia from the Dutch Airquality Network, but the NIW-map is far more detailed.