Deep geological disposal in a multibarrier cementitious facility is being developed by a number of countries for the safe, long-term disposal of intermediate-level radioactive wastes. Intermediate-level radioactive waste, which dominates the radioactive waste inventory in the United Kingdom on a volumetric basis, is a heterogeneous wasteform that contains organic materials including cellulosic materials, encased in concrete. Under the alkaline conditions expected in the cementitious geological disposal facility (GDF), these materials will undergo abiotic, chemical hydrolysis, and will produce degradation products including isosaccharinic acid (ISA) or gluconic acid (GA) that can form soluble complexes with radionuclides. Alkaliphilic microorganisms sampled from a hyperalkaline site contaminated with lime-kiln waste, were able to degrade cellulosic material (tissue paper) in Ca(OH)2 saturated microcosms at a starting pH of 12. Enzymatic processes in these microcosms caused the production of acetate, acidification of the microcosms and a cessation of ISA production. Enrichment cultures prepared at pH 10 and inoculated with a sediment from the same hyperalkaline site were able to degrade ISA, and couple this degradation process to the reduction of electron acceptors that will dominate as the GDF progresses from an aerobic 'open phase' through nitrate- and Fe(III)-reducing conditions post closure. A strictly alkaliphilic bacterium belonging to the Bacillus genus was isolated from the nitrate-reducing enrichment culture, and was found to degrade a variety of organic molecules that are expected to be found in a cementitious GDF. Detailed investigation into the growth of this bacterium suggested that different mechanisms are involved in the biodegradation of ISA and GA, and that bacterial growth is coupled to a decrease in soluble U(VI) concentrations. This implies that microorganisms could have a role in attenuating the mobility of radionuclides in and around a GDF via (i) the biodegradation of cellulose and cessation of ISA production, (ii) the biodegradation of the ligands (ISA and GA) and (iii) the immobilisation of radionuclides. This should facilitate the reduction of undue pessimism in the long-term performance assessment of suchfacilities.