[en] A model of the global circulation of sulfur isotopes has been established in order to understand the secular changes in the isotope composition of sulfur in sedimentary sulfides, in organic bound sulfur, and in sedimentary sulfates. The model takes into consideration weathering of magmatites and sedimentary sulfur by oxidation of sulfide to sulfate, weathering of sedimentary sulfates, formation of sedimentary sulfides and organic bound sulfur, the metamorphosis of sediments, as well as the transfer of sedimentary sulfur into oceans through volcanic processes. It is shown that only the isotope effects in the microbial reduction of sulfate to sulfide, in the assimilation of sulfates by plants, and in weathering of both sedimentary sulfides and organic bound sulfur by the oxidation of sulfide to sulfate may have an influence on the global isotope composition of sulfur in sediments. By the interaction of these processes the secular changes of delta³⁴S values, particularly the characteristic fluctuations, occur. The oceans and warm climate promote the microbial reduction of sulfates to sulfides in the oceans and lead to an increase or a decrease of delta ³⁴S values in sedimentary sulfides as well as in organic bound sulfur and in sedimentary sulfates, respectively. Regression of oceans and cold climate, particularly glaciation, support weathering and oxidation of sulfides to sulfates, and lead to an increase or a decrease of delta³⁴S values in sedimentary sulfides as well as in organic bound sulfur and in sedimentary sulfates, respectively. (author)