Gypsum incrustations on 18 important historical monuments in Prague were investigated from the point of view of their sulfur isotope composition. The ration of ³⁴S and ³²S in deposited sulfates expressed in the delta units (delta ³⁴S) is almost constant for every monument, but a significant difference occurs between monuments in the center of the city and those located on the surrounding hills. Two sets of climatic conditions in Prague have been found where different stone corrosion mechanisms occur. The enrichment of sulfate incrustation in ³⁴S at the city center is explained in terms of the catalytic effects of other pollutants and in difference of corrosion rates.