This paper presents a magnetic resonance imaging (MRI) method for quantifying the spatial concentrations of the paramagnetic species Mn2+ and Mn3+, during pattern formation, in the manganese-catalyzed 1,4-cyclohexanedione-acid-bromate reaction. The relaxivity of each species was measured, which enabled their concentration to be calculated from nuclear magnetic resonance T2 relaxation times. Images were acquired using the fast spin-echo imaging method RARE. By determining the relation, in these experiments, between signal intensity and T2 relaxation time, it was possible to produce T2 maps from the images. These T2 maps were then used to produce concentration maps for the oxidized (Mn3+) and reduced (Mn2+) states of the catalyst.