Liquid velocity and mixing were measured in two concentric tube airlift reactors (ALR) of 30 and 300 dm(3) (nominal volume). The influences of the geometrical design and the reactor scale were studied as a function of gas flow rates. The mixing in the 30 dm(3) ALR, which had an enlarged cross-sectional area in the gas separator region, indicates that in this geometrical configuration most of the mixing occurs in this region. It is demonstrated that the location of the injection and measuring points influence the measurement of mixing time in airlift reactors. Correlations for pressure drop at the bottom, gas hold-up and mass transfer coefficients, which were published in a previous paper, are extended to include the effect of the ratio of downcomer-to-riser cross-sectional areas.