A LIF line scan system was used to obtain unobtrusive, angle-resolved concentration measurements in the vicinity of four forms of impeller agitating a continuously operated stirred tank. The concentration measurements for the two disc turbines, the Rushton and the 'bucket' impeller revealed strong periodic variations in the radial discharge jet. The measurements for the 'bucket' impeller showed that trailing vortex structures are also generated by curved blades and the trailing vortices of both impellers caused undesired regions of high concentration gradients and fluctuations between blade passages. In the case of the 45- and 60 degrees-pitched blade impeller, periodic concentration variations occurred only in the axial discharge of the impellers and concentration gradients and fluctuations were high close to the blades; this was linked to mean velocity variations between a blade-to-blade passage.