Abstract: A one -dimensional two -fluid model for air -lift reactors (ALR) was developed. The model takes into account the variation of gas density due to expansion in the reactor, veloctity slip and interaction between gas and liquid. Numerical solution of the model equations leads to the main characteristics of fluid flow such as liquid velocity and gas holdup. A method for estimating downcomer gas holdup was proposed. Experimental data on both external and internal ALRs were compared with the theoretical prediction and the agreement was found rather satisfactory.