In processes with immobilized cells mass transfer across the boundary layer surrounding the support often plays an important role. Relatively little is known about external mass transfer as a function of the superficial gas velocity in bioreactors such as air-lift loop reactors. In this work ion-exchange resins were used as a solid phase to determine the mass-transfer coefficient in such a reactor. Relations between the Sherwood number and the superficial gas velocity were derived and compared with relations from the literature. Relations in which the Sherwood number is a function of the energy-dissipation rate and relations in which the relative particle velocity is calculated from the rate of free fall of the particle were compared. It was shown that the Sherwood numbers that were functions of the energy-dissipation rates were higher than could be calculated on the basis of the rate of free fall. The Sherwood number obtained was used to calculate the k(1,s) of carrageenan gel beads as a solid phase in an air-lift loop reactor.