The volumetric gas-liquid mass transfer coefficient (k(L)a) was measured for low- and medium-consistency pulp suspensions using the cobalt-catalyzed sulfite oxidation technique. Mass transfer rates were measured in a high-shear mixer far a range of operating parameters, including the rotor speed (N = 10 to 50 rev/s), gas void fraction (X-g = 0.10 to 0.40) and fibre mass concentration (C-m = 0.0 to 0.10). k(L)a measurements were compared with the macroscale flow regime in the vessel (characterized using photographic techniques) and correlated with energy dissipation, gas void fraction and suspension mass concentration in the mixer. We found that gas-liquid mass transfer was significantly reduced in pulp suspensions, even for low suspension concentrations. Part of this reduction was associated with dissolved components leached from the fibres into the liquid phase. This could account for reductions in k(L)a of up to 30% when compared with distilled water. The fibres further reduced k(L)a, with the magnitude of the decrease depending on the fibre mass concentration. Correlations were developed for k(L)a and compared with results available in the literature.