Gas-liquid mass transfer in pulp fibre suspensions in a batch-operated bubble column is explained by observations of bubble size and shape made in a 2D column Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical-capped/dimpled-elliptical bubbles At relatively low mass concentrations (C-m = 2-3% for the softwood and C-m congruent to 7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension Instead, a network of channels formed through which gas flowed In the bubble column, the volumetric gas-liquid mass transfer rate, k(L)a, decreased with increasing suspension concentration From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column A minimum in k(L)a occurred between C-m = 2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed