pH values of the bark of the common tree species black locust (Robinia pseudoacacia L.), sycamore maple (Acer pseudoplatanus L.), European yew (Taxus baccata L.), and European ash (Fraxinus excelsior L.) were evaluated to determine the acid gaseous pollution impact on their bark in eight heavily industrialized cities of southern Poland and relatively unpolluted areas of the Beskidy Mountains, Częstochowa Upland, and Nida Basin. It has been stated that the correlation between SO₂ levels in the atmosphere and the reaction of tree bark exists in all investigated tree species. Hence, the reaction of the bark of these species could be used as a simple indicator of air pollution. The results suggest that the European ash bark could be the best bioindicator.