A method of treating data acquired with the planar laser-induced fluorescence technique has been developed to visualize the topology of two-dimensional concentration fields and to describe the dynamics of the coherent mixing structures identified; This method is based on a conditional binary transformation of the local concentration data, combined with a joint probability calculation. The methodology has been used to investigate the mixing in a stirred tank, at two injection port locations (in the bulk and in the impeller stream region). With bulk injection, a "folding phenomenon" of the coherent mixing structure was detected. Away from this port, large-scale spatially periodic motion was identified, with a characteristic time of oscillation of the order of 2 to 3 s. With injection in the impeller stream region, no spatial instabilities of the coherent structure were detected. Local oscillations of the coherent mixing structure were found both on short and long time-scales (i.e., similar to 1 and similar to 80 s).