Processing of oil sands ores to extract bitumen generates large volumes of tailings which are deposited into large settling basins, where the solids settle by gravity over 3-4 years to become mature fine tailings (MFT). Methanogenesis has been correlated with increased water recovery from and densification of MFT. This phenomenon offers potential tailings management options, including biodensification, an accelerated dewatering process where the microbial community is stimulated by amending MFT with carbon-substrates. The chemistry of methanogenic accelerated dewatering was investigated with 2L and 50L settling columns of MFT amended with carbon-substrates to infer possible mechanisms. It was found that enhanced biogenic gas production induced chemical changes in MFT. The carbon-amended MFT had increased pore-water concentrations of HCO3-, Ca2+, Mg2+ and a lower pH. The pore-water chemistry affects the colloidal properties of the suspended clays in the MFT which leads to the accelerated settling of clay particles and dewatering of MFT.