Microbial sulfate reduction (MSR) is the predominant pathway of organic matter (OM) degradation for half of all microbial cells in the ocean. However, measuring the rate and magnitude of MSR using well established radiotracer 35S-SO42- is limited logistically in the field by regulations on radioactive substances. This study lays the framework for the use of novel stable minor isotope 33S labeled sulfate for in situ measurements of MSR and fractionations. The first stage of this study compares sulfate reduction rates of 33SO42- to those of 35SO42- by pure culture Desulfovibrio vulgaris Hildenborough (DvH) in parallel bottle incubations. The second half of this study focuses on parallel incubations of 33SO42- and 3SO42- in sediment cores from the sub-oxic Santa Barbara Basin (SBB) to examine SRR and model sulfur isotope compositions of newly formed sulfides at the surface where microbial activity and pyrite formation are most active. The conclusions of these two studies pave the way for the eventual use of 33SO42- for in situ experiments where MSR is occurring rapidly and in large magnitudes.