The results of the annual bosgezondheidsinventaris in the Flemish Region shows that are most studied species symptoms of vitality loss. The true causes of the fixed gesture weakening party however can not be inferred from this study. To get a better understanding of the cause-effect relationships regarding bosverzwakking and the impact of air pollution, intensive monitoring is carried out in the woods in the Flemish Region. This project takes place in implementation of EU regulation Forest Focus and fits well in the international cooperation ICP Forests by the United Nations. The characterization of the contaminating deposits from the precipitate (free-field water, throughfall and stamafvloei) and the composition of the soil solution is one of the main characteristics of this research, and is performed in five forest areas, viz. Wijnendalestraat, Gontrode, Zoniënwoud, Brasschaat and Ravel's. This agreement concerns the required parameters to be determined in the context of the aforementioned EU regulation.