The results of an investigation on eight crustose lichen species and one foliose species grown on volcanic tuff from the area of Civita Bagnoregio (central Italy) are given. The observations of thin sections coupled with electron microprobe and x-ray diffraction analysis (XRD) allowed the identification and localization of weathering products produced by lichens. The most important effect of the lichen growth is the removal of calcium from the substrate to form weddellite accumulated in the thalli in different quantities and location depending on the species involved. Fungal hyphae penetrate deeply into the tuff. The relationship between the fungal hyphae and rock often causes the detachment of several millimeters of stone under the central part of the thallus.