Gas hold-up and liquid circulation velocity measurements were made using a 167 dm(3) external loop airlift reactor. The gas-separator was of the open channel configuration. The reactor height was 2.5m with riser and downcomer diameters of 0.19m and 0.14m respectively. The systems investigated were Newtonian air-water and air-glycerol with the superficial air velocity varying between 0.02 and 0.12ms(-1). The ratio of the liquid volume in the gas-separator to the liquid volume in the reactor (volume-ratio) was varied from 0.0% to 37%, to find its minimum critical value for optimum operation of the airlift reactor. For the air-water system, discernible effects of the volume-ratio on riser and, downcomer gas hold-ups and liquid circulation velocity were observed at volume ratios less than or equal to 7%. Beyond this value, the volume-ratio had no effect. For a viscous and foaming air-glycerol system the critical volume-ratio was increased to 19%. New and simple correlations for predicting gas hold-up in the riser, gas hold-up in the downcomer, and Liquid circulation velocity were developed with reasonable accuracy.