In a sulfate reducing process, increasing loading rates and sulfide accumulation may induce population changes resulting in decreasing effectiveness of the process. Thus, the relationship between microbial metabolism changes and population dynamics was studied. An upflow anaerobic sludge blanket reactor was operated at different sulfate loading rates (SLR), from 290 to 981 mg SO4 − S/L d at a constant carbon/sulfur ratio of 0.75. When the SLR was increased, the total organic carbon and sulfate consumption efficiencies decreased to nearly 30% and 25%, respectively. The acetate and propionate yields increased with increasing SLR and 385 ± 7 mg sulfide-S/L d was reached. The ecological indices, determined by random amplified polymorphic DNA and denaturing gradient gel electrophoresis techniques, diversity and evenness were found to be constant, and similarity coefficient values remained higher than 76%. The results suggest that the microbial population changes were negligible compared with metabolic changes when SLR was increased. The sulfide accumulation did not modify the microbial diversity. The sequencing of 16S rRNA genes showed strains related to sulfate reducing, fermentation, and methanogenesis processes. The results indicated that the decreasing of effectiveness, under the experimental conditions tested, was dependent more on operational parameters than microbial changes.