In this study, biochemical methane potential (BMP) tests were conducted to investigate the effect of pH control on the co-digestion of sewage sludge (SS) and sugar beet pulp lixiviation (SBPL) at mesophilic range (35 degrees C). Microbial concentrations (Eubacteria and methanogenic Archaea) are linked to traditional parameters, biogas production and total volatile solids (TVS) removal. Also, the relationship between Eubacteria and Archaea has been analysed. Organic matter being equal, higher net methane generation was reported for the assay with pH adjustment at the beginning of the biochemical methane potential (BMP) test. This showed that there was inhibition of methane generation as measured by the BMP test in the absence of pH adjustment. Evidence of this inhibition was also supported by the methane yield and TVS removal data. Microbial populations in the reactor at the end of both assays were composed of Eubacteria and Archaea, with a higher proportion of Eubacteria in all cases. It is noteworthy that in test 1, when inhibition occurred due to a system pH that was not optimal for the activity of methanogenic Archaea, the analysis showed the largest number of Archaea present. In terms of productivity, it can be said that methanogenic Archaea were inactive. (C) 2014 Elsevier B.V. All rights reserved.