Bitumen extraction from oil sands has resulted in large tailings ponds, containing suspended material that requires over one hundred years to densify. The mature fine tailings (MFT) have become anaerobic and bubbles of gas are observed on the pond surface. Gypsum has been proposed as an additive to increase the rate of MFT consolidation. In a laboratory study, MFT was amended with sulfate and monitored. Pore water sulfate concentrations declined and bicarbonate concentration increased. Nitrate was depleted within 36 d and the levels of soluble iron remained below 0.8 mg L(-1). Thiosulfate and sulfide were detected only near the end of the experiment. Acid volatile sulfides (AVS) increased until day 39, and then reached a plateau. Methane was not detected throughout the incubation in samples amended with sulfate. The increase in AVS supports sulfide incorporation into the solid phase, however, the plateau after 39 d suggests a secondary fate of reduced sulfide.