The aim of this study is to develop a simple model to predict two main parameters of the hydrodynamics of rectangular air-lift reactors (ALR): liquid velocity and gas hold-up. The mathematical model based on mass and momentum balance permit to predict these two parameters with a good accuracy. The experiments used for the validation of the model are carried out in three rectangular ALR of different scales, 4.7 x 10(-3), 0.65 and 4.6 m(3).