Gas holdup and circulation velocity in a rectangular, pilot-scale air-lift reactor were studied as a function of superficial gas velocity. As the gas velocity increases, the gas holdup in the riser and downcomer increase, although the gas distribution in the downcomer remains quite inhomogeneous even at the highest gas velocity studied. Also, as the superficial velocity increases, the distribution of gas in the reactor becomes more homogeneous, with the difference in holdup between the riser and downcomer approaching an asymptotic value. Liquid circulation velocity was estimated by fitting batch tracer data to an N tanks-in-series recirculation model. Liquid circulation velocity increases with the superficial gas velocity and approaches a constant value, which corresponds to an asymptotic difference in holdup between the riser and downcomer. Measured liquid circulation velocity compared favorably with a model of Chisti et al.