In Belgium, Boom Clay is considered a potential host rock for the deep geological disposal of nuclear waste such as Eurobitum, an intermediate-level nitrate-containing radioactive waste form. The presence of dissolved organic matter (DOM) in the Boom Clay may play an important role in the mobility of radionuclides due to complexation and/or reduction of redox-sensitive radionuclides. The biodegradation of this DOM could therefore affect the barrier function of the Boom Clay. Due to the presence of nitrate leaching from Eurobitum into the clay formation, the biodegradation of DOM by a nitrate-reducing population cannot be ruled out in the Boom Clay surrounding a Eurobitum repository. In this paper, the biodegradability of DOM in Boom Clay pore water (BCPW) under nitrate-reducing conditions was investigated in the presence or absence of additional nutrients (i.e., acetate and phosphate) expected to be present in the clay surrounding a Eurobitum repository. The results of these experiments indicate that microbial nitrate reduction and the related degradation of DOM are performed by a syntrophic microbial community but are characterised by very slow kinetics. Nitrate-reducing microorganisms seem to be (at least partially) dependent on fermentation products such as acetate provided by fermenting microorganisms. The observed slow biodegradation of DOM appears to be linked to the characteristics of Boom Clay DOM, rather than to a limitation of nutrients necessary for microbial growth. The addition of phosphate only boosts the microbial growth of certain microbial species but not the biodegradation of DOM. On the other hand, acetate significantly enhances the microbial degradation of DOM. This result can be attributed to the use of acetate as an efficient C source for certain DOM-degrading microbial species and/or as a co-substrate to stimulate DOM degradation.