The objective of this study is to characterize mass transfer in a rectangular air-lift loop reactor in two-phase flow. In a previous work, it has been shown that the reactor presents a complex gas flow pattern. Therefore, first, the global mass transfer volumetric coefficient k(L)a was measured in two-phase flow, by three methods (two based on the liquid phase mass balance, one based on the gas phase mass balance). Then, second, a localized analysis was implemented in order to obtain more information about the phenomena governing the gas phase flow.