The residence time distribution (RTD) analysis of liquid phase was performed in a concentric-tube airlift reactor of 0.07 m(3) nominal volume, regarded as a simple unit and discriminating its different sections (riser, downcomer, bottom zone and gas-liquid separator) using the tracer response technique. The reactor was operated in biphasic continuous flow of liquid and gaseous phase. The volumetric liquid inflow rate, Q(1), and gas superficial velocity in the riser, nu(SGR), were chosen as independent variables. RTD functions in the reactor, namely: the RTD distribution functions at the reactor exit, inclusively in normalized form, E(theta), the distribution ages inside the reactor, I(theta), the intensity of distribution, lambda(theta), respectively, as well as the mean liquid residence time, and the variance of distributions were essentially used for liquid flow diagnosis in the reactor in two operational modes: with and without recirculation. The experimental results revealed that the investigated airlift reactor had a more uniform flow than the tubular and bubble column reactors and the flow defects proved a significant attenuation. These conclusions were confirmed by the single parameter flow models: axial dispersion and tank-in-series models. This fact has a great practical importance, especially in the biotechnological applications of the airlift reactors, where the aerobic cultures can be affected greatly by the flow deficiencies.