Hydrodynamics and mass transfer in internal and external flow airlift reactors have been studied in this work, Gas hold-ups and liquid velocities have been determined in a concentric tube airlift reactor, while oxygen mass transfer has been measured in an external loop airlift reactor. First, tracer measurements were used to verify a conventional axial dispersion model. Further, a model derived from multi-phase Navier-Stokes equations was verified with experimental data from the oxygen mass transfer measurements. Both models were in a good agreement with the measurements.