This paper reports on the experimental investigation carried out to evaluate the interfacial area, a, and liquid-side volumetric mass transfer coefficient, k(L)a, to characterize a multi-stage external loop airlift reactor (with contraction and expansion disks), which has been conceived, designed, and fabricated as an equipment to remove trace pollutants from wastewater. In addition, it has versatile use as a gas-liquid contactor in chemical process industries. In the experimental range of gas and liquid flows investigated, the maximum interfacial area and volumetric mass transfer coefficient obtained was in the order of 300 m(2)/m(3) and 0.05 s(-1), respectively. Correlations developed for predicting the interfacial area and liquid-side volumetric mass transfer coefficient have been found to be encouraging and highly significant from statistical analysis. (C) 2007 Elsevier B.V. All rights reserved.