Titrimetry is a well-established technique that has not been previously used in bioreactors for waste gas treatment. This work explored the information behind titrimetric data such as the proton production (HP) and the proton production rate (HPR) to evaluate their suitability for the on-line monitoring of the biological activity of an aerobic biotrickling filter (BTF) targeting the removal of high concentrations of H2S. The different contributions to the titration rate in the bioreactor were identified and quantified to further calculate the biological HP and HPR due to biological sulfide oxidation. Variable H2S concentrations ranging from 2000 to 10,000 ppm(v) were treated in a lab-scale biotrickling filter mimicking the conditions typically found during biogas desulfurization. Results indicated that the biological HP and HPR are suitable variables for process monitoring since both provided a good correlation between operational conditions and biological activity at inlet H2S concentrations above 4000 ppm(v). The different dynamics of the liquid and the gas phases in BTFs were identified as the main factors influencing the potential use of on-line titrimetry in desulfurizing BTFs. Titrimetric variables did not correlated well with H2S measurements. Instead, titrimetry allowed determining the biological activity leading to key operational changes such as the breakthrough point in which the BTF mainly produced elemental sulfur, a solid that handicaps BTFs operation in the long run due to its accumulation in the packed bed. Titrimetric variables monitoring enhanced the understanding of biological processes occurring in the BTF denoting a great potential to be included as a complementary monitoring tool in the operation of full-scale BTFs for biogas desulfurization. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.