Pressure drops, gas holdup, and mass-transfer coefficients were measured in two concentric-tube airlift reactors of 30 and 300 L (nominal volume). The aspect ratio and the ratio of riser to downcomer cross section were the same for both reactors, but the geometry of the gas separation section was different. The influence of the bottom and top clearances was studied using water and carboxymethyl cellulose solutions and covering a range of effective viscosity from 10(-3) to 25 x 10(-3) Pa.s. The pressure drop results expressed as a Euler number were satisfactorily correlated with Fr, Ga, and a bubble disengagement group M. Correlations are presented also for the gas holdup in the riser, downcomer, and gas separator. The last-mentioned coincides with the correlation for the total holdup in the reactor. The gas-liquid mass-transfer coefficients for all the liquids and geometric variables in both reactors were successfully correlated as Sherwood numbers.