The results of experimental investigations and digital simulation of hydrodynamics of a heterogeneous air-lift reactor have been presented. In the experiments the method disturbance-response has been used. The method allows one to obtain curves which represent changes of the tracer concentration as a function of time at selected places in the vessel. The recognition of the residence time distribution allows one to determine circulation ratios for various hydrodynamic conditions as well as fitting the theoretical model of hydrodynamics of the reactor to real character of the flow.