The two-phase flow field in an axially stirred vessel has been calculated using the commercially available CFD-code CFX4. The multi-fluid approach was used with separate k and epsilon equations solved for each phase. The results were compared with experimental phase-Doppler anemometry data. Particular attention was given to the distribution of slip velocities in the tank. Qualitative agreement between calculations and experimental data was obtained for the axial component of the slip and for the turbulence kinetic energy difference between the phases. The radial and tangential components of the slip vector were greatly under-predicted in the calculations. The drag was found to be most important of the investigated inter-phase momentum transfer terms. Four different drag models were tested and found to give very similar results. Finally, the importance of the slip velocity as well as the particle velocity for the mass transfer in stirred tanks was discussed.