Oxygen isotope compositions of dissolved sulfate in pore waters at open-ocean and equatorial upwelling sites drilled during Leg 201 indicate the presence of microbial sulfur transformations in the deep-sea sediments of Sites 1225 and 1226 to depths of 418 mbsf and clearly reflect different degrees of MSR activity in sediments. The open-ocean Site 1231 had 18O-SO4 values that were close to normal seawater values (~10) throughout the entire ~114-m sediment column and showed no evidence for MSR, consistent with chemical compositions of pore water and low organic matter contents. Evolution of 18O-SO4 values and MSR activity increased progressively from Site 1225 to Site 1226, reaching maximum values of +20 and +28, respectively. Combined sulfate concentration and 18O-SO4 data at Sites 1225 and 1226 suggest the presence of a sulfate-reducing microbial community and oxidation of sulfide with further disproportionation of sulfur intermediates. Results from this study show that the isotopic composition of oxygen in pore water sulfate is an important tool in revealing processes of dynamic sulfur transformations by the biosphere of deep-sea sediments.