Two classes of biochemical inhibitors of sulfate reduction were tested in batch and continuous anaerobic digesters. At high phosphate concentrations, molybdate was an effective and selective inhibitor of sulfate reduction in fed-batch systems, although non-specific effects were observed at lower phosphate concentrations. Transition metal divalent cations were effective and selective inhibitors of sulfate reduction at all conditions tested in fed-batch digesters. In continuous digesters, molybdate inhibited sulfate reduction but also resulted in non-specific inhibition independent of phosphate concentration; transition metals proved to be ineffective in curbing sulfate reduction. Experiments were conducted to explain the non-success of inhibitors in continuous digesters.