Glucose-fed high-rate UASB reactors were tested at three COD/SO4 ratios and hydraulic retention times to promote sulfate reducing activity and observe the effects on reactor performance. Different COD/SO4 ratios (20, 10, and 5) resulted in changes in organic matter removal, methane production, alkalinity, dissolved sulfide and biomass concentrations and profile. The COD removal dropped from 95 to 80-84% at the lowest COD/SO4 ratio. Sulfate was removed at 79 to 89% at the highest ratio and dropped to 72 to 74% with increasing sulfate loading. Alkalinity was produced at higher levels with increasing sulfate loading. Specific methane production dropped with decreasing hydraulic retention times. Sulfate-reducing activity used a maximum of 11.7% of organic matter at the highest sulfate loading level, producing a slight shift to sulfate-reducing activity in the substrate competition between sulfate-reducing bacteria and methanogens. Increased sulfate loading at COD/SO4 ratios of 10 and 5 caused deterioration of the concentration profile of the sludge, resulting in biomass washout and decreased volatile fraction of biosolids in the reactors.