The role of the gas-liquid separator on hydrodynamic characteristics in an internal-loop airlift reactor (ALR) was investigated. Both gas holdup and liquid velocity were measured in a 30 dm(3) airlift reactor with two different head configurations: with and without an enlarged separator. A magnetic tracer method using a neutrally buoyant magnetic particle as flowfollower was used to measure the Liquid velocity in all sections of the internal-loop airlift reactor. Average liquid circulation velocities in tire main parts of the ALR were compared for both reactor configurations. At low air flow rates the separator had no influence on gas holdup, circulation velocity and intensity of turbulence in the downcomer and separator, At higher superficial air velocities, however, the separator design had a decisive effect an the hydrodynamic parameters in the downcomer and the separator, On the ether hand, the gas holdup in the riser was only slightly influenced by the separator configuration in the whole range of air flew. Circulation Wow regimes, characterising tile behaviour of bubbles in the downcomer, were identified and the effect of the separator on these regimes was assessed.