Gas holdup and superficial liquid velocity in the downcomer and riser are studied for an external loop airlift reactor with a downcomer-to-riser area ratio of 1:16. Two downcomer configurations are investigated over a range of superficial gas velocities (0.5 ≤ UG ≤ 20 cm/s) using three aeration plate open area ratios (A = 0.62, 0.99, and 2.22%). These results are compared to a bubble column operated with similar operating conditions. Gas holdup in both the riser and downcomer are found to increase with increasing superficial gas velocity. Results show that riser gas holdup varies slightly with downcomer configuration, while a considerable variation is observed for downcomer gas holdup. The superficial liquid velocity varies considerably for the two downcomer configurations and is a function of superficial gas velocity and flow conditions in the downcomer. Observed variations are independent of aeration plate open area ratio.