The rate of mass transfer from the gas to water phases was measured in a commercial, high-shear, laboratory mixer under conditions typical of medium-consistency bleaching. The gas-liquid volumetric mass transfer coefficient, k(L)a, was measured using the cobalt-catalyzed sulfite oxidation technique. Suspensions of fully-bleached kraft pulp and synthetic nylon fibres were used, with mass transfer rates measured over a range of suspension compositions and mixer operating conditions. In the presence of pulp fibre, mass transfer rates were significantly reduced over the comparable water cases. The same dramatic decrease in mass transfer was not observed for the nylon suspensions, although k(L)a did decrease with increasing suspension concentration. Comparison of this data with that obtained from ozone bleaching experiments confirmed that at medium-consistency gas-liquid mass transfer controls ozone bleaching.