Mixing of an inert marker added to the bulk region of a Rushton-turbine-agitated reactor was investigated by planar laser-induced fluorescence. The effect of the turbine agitation speed on the mixing process was initially studied statistically. Average concentration fields, calculated at several sections of the reactor, revealed the general spreading of the contaminant. This approach, however, was found to be unsatisfactory in studying the mechanisms of macro-mixing in the reactor because of the strong intermittence in the flow. Therefore, the concentration data were analysed using conditional techniques, namely the dynamic structural concept, so as to identify the presence of coherent mixing structures and characterise them in space. Instability patterns were observed with increasing turbine agitation speed from 0.3 to 0.7 W/m(3). Using geometrical concepts, the mechanisms of this spatial instability were further described. General conclusions on the spreading of the contaminant were drawn on the basis of the how instability with respect to transport by convection and diffusion phenomena.