The mixing of two water feedstreams is investigated in a 0.02 m(3) continuous stirred tank, from concentration measurements. These measurements are carried out with a laser sheet in a very thin vertical plane through the vessel tangent to the impeller. The proportionality between the grey level obtained by planar laser induced fluorescence and the local concentration has been previously determined. The fields of instantaneous concentration, temporal variance and mixing index are processed from the grey level images.