Biochemical weathering mechanisms carried out by Penicillium frequentans and Cladosporium cladosporoides on unaltered sandstone, granite and limestone were studied using FTIR, X-ray diffraction, atomic absorption and flame photometry. Strains belonging to both fungal species, isolated from the facades of two Spanish Cathedrals, were used. Large amounts of oxalic, citric and gluconic acids were produced by P. frequentans in broth cultures. These metabolites caused extensive deterioration of clay silicates, micas and feldspars from both sandstone and granite and also of calcite and dolomite from limestone, as a result of high cation release and organic salts formation such as calcium, magnesium and ferric oxalates and calcium citrates. Comparatively, the biodegradative effect brought about by C. cladosporoides was much less than that caused by P. frequentans. Neither organic acids nor organic salts were formed by C. cladosporiodes samples. It is concluded that filamentous fungi are able to cause an extensive weathering of stone, due principally to organic acid- excretion, although other metabolites participate to a lesser extent in these deteriorative processes. Ecological adaptative mechanisms, such micronutrients uptake and trivalent cations chelation (Fe3+ and Al3+) are derived from fungal growth on stone monuments.