A comprehensive study on the liquid mixing with/without instantaneous reactions was conducted using planar laser induced fluorescence (PLIF) technique, where a novel reactive PLIF technique was adopted to quantitatively visualize the dynamic variation of the concentration of fluorescence dye due to signal quenching by a Fenton reaction. The factors (e.g., tracer injection direction, tracer injection position, impeller speed) influencing the reactive and non-reactive mixing processes were investigated, and the mixing performances were characterized by non-reactive mixing times (tau(95) and tau(99)), and reactive mixing time (theta(99)), respectively. The experimental results from the 2-D measurements of liquid mixing behavior emphasized the significance of the understanding on the spatio-temporal mixing patterns: not only did the tracer injection position affect the mixing performances in the stirred tank, but also the sampling position to monitor the mixing status would lead to a large difference in evaluating the mixing performance. For the stirred tank with fast reactions, the reactive mixing performance had a strong interplay with the non-reactive mixing. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.