Experimental results relating to the hydrodynamic behaviour of an impinging-stream reactor are presented and discussed. The parameters varied were the gas flow rate and the liquid flow rate while the quantities measured were the gas holdup, the mean bubble diameter and the absolute bubble velocity in the main reactor tube. From these quantities, the internal gas recirculation rate and the gas flow ratio have been determined. The internal gas recirculation rate is important in evaluating the mass-transfer performance of the impinging-stream reactor.