Dispersion of gas in pulp suspensions was investigated downstream of an in-line mechanical mixer for fibre mass concentrations from 0% to 3.0%, superficial liquid/pulp velocities from 0.5 to 3.0 m/s, superficial gas velocities from 0.11 to 0.44 m/s and impeller speeds from 0 to 800 rpm, based on electrical resistance tomography (ERT) and a gas mixing index, derived from the standard deviation of local gas holdup. When buoyancy was less significant, bubbles dispersed uniformly in the high-shear zone just downstream of the impeller, with considerably better mixing than without the impeller. Reflocculation then likely occurred downstream, accelerating bubble coalescence and demixing, especially for higher suspension concentrations. For buoyancy-dominated flow, however, the impeller disrupted the fibre networks, and buoyancy caused the gas to segregate to the top of the pipe, resulting in worse mixing than for no impeller. Gas dispersion improved downstream at higher fibre mass concentrations, likely due to re-forming of fibre networks causing liquid/pulp slugs and elongated bubble flow at the top of the pipe, whereas stratified flow occurred at lower concentrations. Mixing improved with decreasing impeller speed, likely because fibre networks re-established more rapidly. (C) 2013 Elsevier Ltd. All rights reserved.