The Two-Fluid formulation coupled with a k-epsilon turbulence model is used for CFD (Computational Fluid Dynamics) analysis of a gas-liquid flow in an internal-loop airlift reactor. The studied airlift reactor consist of a riser section interconnected at top and bottom with 2 down-comer sections. Several flow regimes can be distinguished (Van Benthum et al., 1999), which all have their characteristic features. Flow oscillations have been measured in these type of geometries at certain regimes of the gas flow-rate. The occurrence of the instabilities showed to be sensitive to the superficial velocity, the free surface height and the location of the inlet. The trend of the predicted dynamical behavior is similar to what was found experimentally: the oscillation period is decreasing with higher gas flow rates.