The hydrodynamic behaviour of airlift reactors strongly depends on the flow regime in the riser. Representing the slip velocity of the gas as a function of the overall fluid velocity (U-gr + U-lr) in the riser of an airlift reactor allows to distinguish between homogeneous and heterogeneous flow regimes. The transition between the flow regimes depends on the resistance to liquid circulation; this resistance has been varied by changing the downcomer diameter and by inserting a valve on the downcomer. The flow regime also strongly depends on the initial gas distribution and on the size of the plant: improving the sparger and increasing the column height lead to a more homogeneous flow. This is confirmed by estimating the parameters of the classic Zuber and Findlay model for the present results and for these of literature.