The hydrodynamic behaviour of an external airlift reactor is modelled using simple elementary models : the liquid now in the riser and the downcomer is represented as plug flow with axial dispersion, while the gas-liquid separator and the bottom junction are considered as CSTR’s for the liquid The system of differential equations resulting from the mass balances applied to the different sections of the reactor are solved in the real time domain using a powerful software (MODEST). The model parameters are evaluated by adjusting the experimental and simulated responses reactor to a tracer impulse injection at six different locations in the riser and the downcomer. A sensitivity analysis shows that the liquid velocities determined this way are much more precisely known than the axial dispersion coefficients. The velocities thus obtained compare well with data measured by a thermal tracer technique.