The volumetric gas-liquid mass transfer rate, k(L)a, was measured under batch conditions in a 0.28 m diameter laboratory-scale retention column. Tests on water, and on unbleached kraft (UBK) pulp suspensions (mass fractions, C-m from 0.013 to 0.09) were made with air or nitrogen sparged through the column at superficial gas velocities between 0.0015 to 0.05 m/s. k(L)a varied with suspension mass concentration and superficial gas velocity, initially decreasing with increasing mass concentration, reaching a minimum between C-m = 0.03 and 0.06, and then increasing. The minimum in k(L)a coincided with a change in hydrodynamics within the column, from bubble column behaviour below C-m = 0.03 to porous solid behaviour above C-m = 0.06.