The phenomenological simulation model, proposed previously by the authors for bubble columns, was extended to examine gas hold-ups and volumetric mass transfer coefficients in external-loop airlift reactors. The proposed simulation model is based on bubble break-up and coalescence - both being primary phenomena in airlift reactors. The characteristic liquid velocity in external-loop airlift reactors is assumed to be the superficial liquid velocity in the riser instead of the liquid velocity at the column axis in bubble columns. The predictions of the proposed phenomenological simulation model compare favourably with the present experimental results and the data published in the literature.