Despite the increasing importance of air-lift reactors, very little published information is available on how the ratio of aerated to non-aerated cross-sectional areas (A(d)/A(r)) of the reactor, and the aerating system affect the oxygen mass transfer and other performance characteristics. The influence of the ratio of cross-sectional areas of downcomer to riser (A(d)/A(r)), and the type of gas-distributor on volumetric oxygen mass-transfer coefficient, gas holdup, and liquid circulation velocity were investigated in a 114 L rectangular air-lift reactor with motile baffle, separating aerated from non-aerated compartments. The results indicated that the oxygen mass transfer characteristics of rectangular air-lift reactor are better at A(d)/A(r) of 2.0; the gas holdup and liquid circulation time showed maximum values at the same ratio. A new empirical model for the effects of the aeration rate and A(d)/A(r) ratio was proposed to account for the observed changes of volumetric oxygen mass transfer coefficient. The model took into account the different mixing patterns in the reactor for both cases: A(d)/A(r) <= 1 and A(d)/A(r) > 1 using a wide range of these ratios. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.