The liquid-liquid mixing process coupled with chemical reactions in a mini-scale jet mixer was visualized by reactive laser-induced fluorescence (reactive-LIF) technique for a deep understanding of the interplay between the mixing and the simultaneous reactions. A novel approach for implementing the reactive-LIF measurements was advanced in this work, where the principle was based on the quenching of the fluorescence signal emitted from the Rhodamine-B dye using the mechanism of Fenton reaction. The purely physical mixing and the reactive mixing processes were investigated extensively by comparing the concentration fields under different operating conditions, i.e. the different momentum ratios between the jet and the bulk flows, and the different Reynolds numbers in the mixing channel of a mini-scale mixer. The results revealed that the transient dynamics in the reactive mixing process cannot be precisely understood based on the time-averaged concentration fields from either the physical or the reactive mixing measurements. The coupled mixing and the reaction processes occurred simultaneously, unless the rapid mixing was achieved by tuning the operating conditions so that the processes can be decoupled into two isolated ones. (C) 2009 Elsevier B.V. All rights reserved.