The reacting flow fields in reactive miscible viscous fingering in a Hele-Shaw cell studied by Nagatsu and Ueda had not been completely elucidated, mainly because one cannot exactly recognize where and when the reaction takes place in the reactive fingering pattern. We developed a novel experimental method that allowed its to identify the reaction region in the fingering pattern employed in the previous studies. The novel method involves switching of the less-viscous liquid injected in both the nonreactive and reactive experiments. By using the novel method, we succeeded in showing how, the reaction region in the fingering pattern was affected by the initial reactant concentrations, the Peclet number, and time. We propose physical models of the reacting flow field in the cell's gap direction that can explain the obtained experimental results. (c) 2009 American Institute of Chemical Engineers AIChE J, 55: 563-580, 2009