In the biotechnological gas desulfurization process, dissolved sulfide is oxidized into predominantly elemental sulfur (S8) by sulfide oxidizing bacteria (SOB) under strict oxygen limited conditions. An online measurement of the oxidation reduction potential (ORP) is used to control the O2 supply to the bioreactor to maximize the selectivity for S8 formation and minimize unwanted sulfate formation. While the ORP in the bioreactor is considered to be a measure of mainly sulfide and oxygen concentrations, in practice, none of these components are detectable. In this study, we investigated the sensitivity of stored charge in SOB toward ORP. Stored charge in SOB was measured in an electrochemical cell. These SOB were harvested from the bioreactor of a pilot-scale desulfurization installation. The bioreactor, in which sulfide was not detectable, was operated at different ORP setpoints (−250 to −390 mV vs Ag/AgCl). It was found that more charge was recovered from SOB when the ORP in the aerated bioreactor was lower. These measurements were used to calibrate a model to describe the ORP based on charge storage in SOB, which showed that S8 formation increases and sulfate formation decreases when SOB contain more charge. This can be used to further optimize the biotechnological desulfurization process.