Dispersion of gas into pulp-suspension horizontal flow was investigated downstream of 900 tees for ranges of fibre mass concentrations (0-3.0%), superficial liquid/pulp velocities (0.5-5.0 m/s) and superficial gas velocities (0.11-0.44 m/s) based on a gas mixing index, derived from the standard deviation of cross-sectional local gas holdup obtained from electrical resistance tomography. Mixing for dilute suspensions was similar to that for water, but differed significantly for higher suspension concentrations. Mixing worsened with increasing fibre mass concentration for the bubble flow regime, likely due to dense fibre networks in the core of the pipe causing bubbles to congregate near the wall. When buoyancy was significant, gas uniformity improved with increasing pulp concentration, since robust fibre networks caused liquid/pulp slugs to flow at the top of the pipe, whereas stratified flow was approached at lower concentrations. Mixing was less dependent on superficial liquid/pulp velocity at higher pulp concentrations, due to less variation in flow regimes. (C) 2012 Elsevier Ltd. All rights reserved.