Gas-liquid mass transfer behavior in an external loop airlift reactor is investigated. The riser and the downcomer of 6 m height have a diameter of 0.15 and 0.08 m, respectively. The classical 'gassing out' dynamic method is used to obtain the evolution with time of dissolved oxygen concentration profiles. An analytical description of the concentration profiles with time following the switch in the inlet gas concentration is presented and allows the evaluation of the gas-liquid mass transfer coefficient for the different gas velocities used. The comparison between the values of k(L)a determined from this simplified model and those obtained in the same reactor from a more complete and sophisticated model shows a good agreement.