Abstract: The gas holdup and linear liquid velocity in an internal loop air - lift reactor (ALR) under a wide range of operating conditions were determined by means of the electrochemical method. The energy equation was derived on the basis of the one-dimensional two - fluid model of ALR presented in Part I of this series. The main characteristic parameters of fluid flow in both internal and external loop reactors, such as liquid velocity, gas holdup and the partition of dissipation of input energy were analysed in detail.