Oscillations in the gas holdup and liquid circulation velocity were found at low liquid levels in an airlift reactor with a designed gas-liquid separator. The hydrodynamic behavior and the frequency and amplitude of the oscillations were investigated. The gas holdup and liquid circulation velocity oscillated at the same frequency. The frequency increased from 0.07 to 0.18 Hz with an increase in the superficial gas velocity or liquid level. The gas holdup oscillation occurred mainly in the downcomer. The oscillation amplitude of the average gas holdup in the downcomer was 0.02 at superficial gas velocities of 0.03-0.06 m/s, and was close to zero at other superficial gas velocities. Using the superficial gas velocity and unaerated liquid level, the flow was classified into three regimes: non-circulating regime, self-oscillation regime and completely circulating regime. The self-oscillation regime, determined from the oscillation amplitude, became wider as the liquid level decreased. (C) 2010 Elsevier B.V. All rights reserved.