Enormous volumes of tailings produced during bitumen extraction from oil sands ores are stored in settling basins/tailings ponds. The current inventory of tailings in northern Alberta, Canada exceeds 850 million m3. Biogenic methane emissions have been observed from the surfaces of tailings ponds and about 40 million L of methane day-1 was estimated from a single tailings pond (Mildred Lake Settling Basin) in 1999. This research project was initiated to investigate the source and mechanism of methane emission from the oil sands tailings ponds. The mature fine tailings (MFT) were collected from Syncrude Canada Ltd. and Shell Albian Sands tailings ponds and investigated for methanogenic biodegradation of solvent hydrocarbons that are used in the bitumen extraction process and the residual fractions of these solvents that are present in the tailings deposited in the tailings ponds. Our laboratory experiments have shown that only short-chain n-alkanes (C6-C10) and certain monoaromatics (BTEX) present in C3-C14 range hydrocarbons entrained in Syncrude tailings are readily biodegraded by the indigenous microorganism in the tailings ponds to produce methane. In contrast, a very long acclimation period has been observed for indigenous microbes to degrade long-chain n-alkanes (C14-18) and branched alkanes such as 2-methylpentane. Experiments are in progress to monitor the degradation of these recalcitrant compounds. The molecular analysis of 16S rRNA genes revealed that different microbial communities are involved in the degradation of different groups of petroleum hydrocarbon in oil sands tailings. Understanding the mechanism(s) of biogenic methane production and predicting emissions from oil sands tailings ponds are important objectives for effective management of tailings and greenhouse gas emissions.