The selective flux of sulfur during magma emplacement is proposed to explain some abnormal δ34S data from granitic and basaltic rocks. It is assumed that on the one hand a quasi-equilibrium isotope fractionation exists between sulfate and sulfide during magma cooling, and on the other hand a non-equilibrium fractionation occurs between the fluxed sulfur and the magmatic sulfur. The results show that at high fO2,34S is preferably enriched in sulfate with decreasing temperature, without a corresponding depletion of sulfide in34S. The δ34S value of solidified rocks is then significantly shifted in the positive direction due to the selective degassing or assimilation. Conversely, at low fO2,34S is preferably depleted in sulfide as temperature declines, while a corresponding34S-enrichment in sulfate does not occur. As a consequence, δ34S value of the rocks is driver in the negative direction.