A B S T R A C TThe subject of this research was electron equivalent fluxes about the decomposition of pharmaceuticals (sulfamethazine and sulfathiazole) using an oxygen-based membrane biofilm reactor (MBfR). The influent concentrations in pharmaceuticals feed-medium were (in ppb): sulfamethazine (40) and sulfathiazole (85). The oxygen-based MBfR system consisted of two membrane modules connected to a recirculation loop. The main membrane module contained a bundle of 32 hydrophobic hollow-fiber membranes inside a polyvinyl-chloride pipe shell and the other module contained a single fiber used to take biofilm samples. Pure O 2 was supplied to the inside of the hollow fibers through the manifold at the base, and the O 2 pressure for both reactors was 13 kPa. (1 kPa = 0.0099 atm = 0.145 psi). HRT was 3 h. The decomposition ratio of pharmaceuticals (sulfamethazine and sulfathiazole) using oxygenbased MBfR was (%): sulfamethazine (77 ± 2) and sulfathiazole (87 ± 2). In all cases, nitrification was the largest provider of electrons, together accounted for at least 99.98% of the total electron flux.