The use of air in backwash of hollow-fiber modules was investigated experimentally from bench to full scale. Modules operated in a dead-end and outside-in mode: they were fouled by either a bentonite suspension or a raw river water and then backwashed in presence of air. The air was injected into the retentate compartment either in combination with a reversed permeate flux or together with feed water after a brief permeate back flow. Results indicate that the cake layer is instantaneously lifted off by the reversed permeate flux and is concentrated in the free volume of the module. To remove it from the module and recover the feed concentration, this volume has to be rinsed with a volume at least three times as big. The air, by its piston-like action, improves material removal and reduces the volume of concentrated foulant to be flushed. So the backwash time is reduced and its efficiency is improved. An optimum air flow rate can be found that is independent of the water flow rate used to flush the module free-volume.