Global hydrodynamic characteristics-gas hold-up and liquid circulating velocity-were measured in two external loop airlift contactors. Geometrical parameters, such as the height of the airlift, the design of the phase separator, the gas sparger and operating parameters, were found to have significant effects on global hydrodynamic characteristics. In particular, circulation velocity was found to be dependent on liquid volume. To predict gas hold-up in the riser, a simple model, which gives good agreement with experimental data obtained with various airlift geometries, was proposed.