Hydrodynamic and mass transfer experiments have been carried out in three geometrically similar external loop airlift reactors. The ratio of the cross-sectional area of the downcomer to riser, i.e., A(d)/A(r) is used as a scale-up factor to distinguish the reactors (i.e., A(d)/A(r) = 0.25, 0.44 and 1.0 for the small, medium and large reactors respectively). At constant gas throughputs, increasing the area ratio results in an increase in liquid circulation velocity and reduction in both gas hold-up and mass transfer coefficient. New empirical correlations which incorporate the most important parameters that affect the performance of airlift reactors have been developed for gas hold-up, liquid circulation velocity and volumetric mass transfer coefficients. The predictions of the proposed correlations compare favourably with the present experimental results for all reactors.