Abstract Earlier work has suggested that the addition of extracellular polymers to the seed sludge of an upflow sludge blanket reactor may inhibit its performance during the initial start‐up phase. Measurements of the specific methanogenic activity using sucrose and acetate/formate substrates, with and without the addition of polymers, showed that there were no inhibitory effects. However, in the absence of a carbonaceous substrate, polymer addition did inhibit the endogenous production of biogas.