Under the alkaline conditions expected in an intermediate-level waste repository, cellulosic material will undergo chemical hydrolysis. This will produce hydrolysis products, some of which can form soluble complexes with some radionuclides. Analyses of samples containing autoclaved tissue and cotton wool incubated in a saturated solution of Ca(OH)2 ( pH > 12) confirmed previous reports that isosaccharinic acid (ISA) is produced from these cellulose polymers at high pH. However, when inoculated with a sediment sample from a hyperalkaline site contaminated with lime-kiln waste, microbial activity was implicated in the enzymatic hydrolysis of cellulose and the subsequent production of acetate. This in turn led to acidification of the microcosms and a marked decrease in ISA production from the abiotic alkali hydrolysis of cellulose. DNA analyses of microbial communities present in the microcosms further support the hypothesis that bacterial activities can have a controlling influence on the formation of organic acids, including ISA, via an interplay between direct and indirect mechanisms. These and previous results imply that microorganisms could have a role in attenuating the mobility of some radionuclides in and around a geological disposal facility, via either the direct biodegradation of ISA or by catalysing cellulose fermentation and therefore preventing the formation of ISA.