A mechanistic model for mixing and mass transfer in both airlift bubble column and loop bioreactors is described. The model is a useful design tool to improve oxygen-transfer characteristics. It is shown that the model fits mixing and mass-transfer experimental data closely. The model confirms that the overall oxygen-transfer coefficient in the riser of a loop airlift fermenter is similar to that in bubble columns and that lower measured transfer rates are due solely to the dead volume in the downcomer. The effects of parameters such as mass-transfer coefficient, aeration rate, dispersion coefficient, sparger rotation speed, and liquid circulation velocity are presented in terms of both the Liquid- and gas-phase oxygen concentrations.