Hydrogenotrophic wastewater denitrification was investigated using a bench-scale hollow-fiber membrane bioreactor (HFMB). In the HFMB, hydrogen (H2) was passed through the lumen of hollow-fiber membranes and nitrified wastewater was supplied to the shell of the reactor. A mass transfer model was developed and found to be a good tool to estimate H2 mass transfer coefficients at varying recirculation velocities. Under steady conditions, effluent NO3(-)-N concentrations less than 10 mg/L were achieved at an empty bed contact time of 8.3 hours when pH and membrane fouling were controlled. An average nitrogen flux of 0.88 g NO3(-) -N/m2 x d was observed. Dissolved oxygen in the influent wastewater did not adversely affect overall nitrogen removal. Under transient conditions, similar to those of onsite processes, overall nitrogen removal efficiencies of 74 to 82% were observed. Confocal laser scanning microscopy revealed that the denitrifying biofilm was loosely associated with the membrane surfaces.