Previous papers on simulated mixing times in stirred vessels using CFD have sometimes given predictions in good agreement with empirical equations based on experiments and some have not. In this study, mixing times have been measured for a vessel agitated by a Rushton turbine and compared with those predicted by CFD. The flow field was developed using the sliding mesh approach and computational parameters and the point of addition of the tracer have been varied. The simulations were very insensitive to the former whilst the radial distance from the wall of the latter had a very profound effect on both the mixing time and the development of the concentration field. When the addition point was close to the sliding mesh surface, the simulation was in good agreement with experimental values and empirical predictions whilst that for a point close to the wall was much too long. This finding may explain the contradictions in the literature.