Based on airlift configuration, a novel circulating jet-loop submerged membrane bioreactor (JLMBR) adapted to ammonium partial oxidation has been developed. Membrane technology and combined air and water forced circulation are adopted to obtain a high biomass retention time and to achieve a separate control of mixing and aeration. This study is intended to determine how gas-liquid mass transfer is affected by operating conditions. In a first approximation, liquid was assumed to be perfectly mixed. A classical non-steady state clean water test, known as the "gas out-gas in" method, was used to determine the gas-liquid mass transfer coefficient k(L)a. Air and recirculated liquid superficial velocities were gradually increased from 0.013 to 0.019m s(-1) and 0.0056 to 0.011 m s(-1), respectively. Subsequently, the gas-liquid mass transfer coefficient kLa varied from 0.01 to 0.02 s(-1). It appears to be influenced by the combined action of air and recirculated liquid flowrates in the range 0.72-1.03N m(3) h(-1) and 0.30-0.58 m(3) h(-1), respectively, for air and liquid. Correlations are proposed to describe this double influence. Experiments were performed on tap water and a culture medium used for the autotrophic growth of nitrifying bacteria, respectively. Oxygen transfer appeared to be not significantly affected by the mineral salt (0.48 g 1(-1)) encountered in this medium. (c) 2005 Elsevier Ltd. All rights reserved.