This work concerns mass transport in anaerobic bio®lms, formed under up¯ow liquidvelocities similar to the prevailing conditions in anaerobic reactors used for wastewater treatment.During bio®lm formation under liquid velocities of 1.5 and 13.2 m/h, internal mass transfer coe cientswere routinely measured. Mass transfer coe cients attained pseudo steady-state values between 2±4 10ÿ3 m/h, no dependence being observed between bulk ¯ow and internal mass transport rates.However, a transient variation in the liquid velocity from 1.5 up to 13.2 m/h, imposed after the bio®lmhad reached the steady-state, increased the internal mass transport by 20% on average. This resultsuggests that periodic changes in the bulk ¯uid velocity can be used as a tool to increase the transportof soluble substrates inside already formed bio®lms, although the e ect seems to be limited.