Bentonites are montmorillonite-based materials being considered for use as engineered barriers during the disposal of high heat generating radioactive wastes. Bentonite has several beneficial properties when it comes to the disposal of nuclear waste, including the ability to swell in the presence of water producing a low permeability, material with amphoteric ion exchange sites able to absorb radionuclides. They also offer the ability to buffer the pH, providing stable conditions for the enclosed canisters. Microorganisms have been identified in natural bentonite horizons and may be viable and active under geological disposal conditions. Initial studies focused on the viability of sulphate-reducing bacteria (SRB) due to their role in the corrosion of canister materials. Fe(III)-reducing bacteria (IRB) were also of interest as Fe(III)-reduction in smectite minerals influences their geochemistry, and ability to swell. Indigenous microbial communities in a range of bentonites were subjected to mechanical pressure, heat, and gamma radiation treatments, and their IRB, and SRB contents counted. All the bentonites contained viable microorganisms after the treatments albeit in low numbers. Cores from a field-scale geological disposal experiment (FEBEX-DP) were also analyzed to see how the temperature gradient influenced the long-term viability of microorganisms. The FEBEX cores tested negative for culturable organisms. However, DNA was extracted, and 16S rRNA genes amplified and sequenced, suggesting an enrichment in anaerobic bacteria at the lower end of the temperature gradient (55 Â°C). The reduction of Fe(III) by the subsurface bacterium G. sulfurreducens was quantified in SWy-2 montmorillonite and FEBEX bentonite. A significant degree of biological Fe(III)-reduction was observed in the SWy-2 montmorillonite (81 % reduced). Meanwhile, the FEBEX bentonite was largely recalcitrant to biological Fe(III)-reduction (17 % reduced). The lack of Fe(III)-reduction in the FEBEX bentonite was attributed to the presence of iron oxides coating the montmorillonite, or potentially a lower swelling ability which decreased the exposure of Fe(III) in the bentonite to G. sulfurreducens cells. The Fe(III)-reduction in the SWy-2 montmorillonite increased the cation exchange capacity (CEC), which could influence the swelling pressure generated by the bentonite. The role of swelling pressure in restricting microbial Fe(III)-reduction in the SWy-3 montmorillonite was also investigated, and suggested a wet density of over 1900 kg m-3 (1411 kg m-3 dry density) in SWy-3 montmorillonite would be required to halt biological Fe(III)-reduction, with considerable activity observed at 1750 kg m-3 (1176 kg m-3 dry density) and below. Fe(III)-reduction in SWy-2 montmorillonite also affected the solubility of selenium oxyanions. Selenite and selenate oxyanions were both reduced and precipitated, selenate as a result of direct biological reduction with G. sulfurreducens cells, and selenate via a chemical reaction with the montmorillonite. The results of the molecular ecology on the FEBEX in-situ cores, combined with data from the IRB swelling pressure experiments suggest that microbial activity will be inhibited if a sufficient bentonite buffer density is maintained. If the bentonite density decreases, then microbial activity might be restored, most likely by spore-forming bacteria. In the case of IRB, biological Fe(III)-reduction could cause further swelling pressure decreases. However, IRB may also promote the capture of selenium oxyanions, and potentially other radionuclide species, minimizing their migration from a geodisposal barrier system.