dMass transfer is calculated, for air-water systems, using measurements from a dissolved oxygen probe with a simple model. Most measurements are taken at the 20 litre scale, but successful mass transfer measurements have also been made at the 5000 litre scale. The full-scale designs are illustrated for a number of different processes, and examples given of the reduction in cycle-time achievable by a change of impeller configuration. The effects of geometry and gassed-power input on the mass transfer coefficient are also discussed.