In numerous freshwater and marine environments, ferrous iron (Fe2+) can react with manganese oxides in a redox reaction. However, there are few quantitative data describing reaction rates. A “soluble” (colloidal and nanoparticulate) phase manganese dioxide (MnO2) was used to obtain kinetic data on MnO2 reduction by Fe2+ with a stopped flow UV–Vis method. Stopped flow experiments were carried out in unbuffered solutions between pH 4.9 and 5.36 and also at pH 7. The reaction was determined to be first order with respect to MnO2 and Fe2+ and second order overall. It is important to subtract absorbance of Fe(III) products from the UV–Vis data and to acquire data from the first milliseconds of the reaction. After subtracting Fe(III) product absorbance, the average second-order rate constant was determined to be 4338 ± 249 M−1 s−1 at 25 °C and pH 5. Reactions of 5 μM MnO2 with 50 and 100 μM Fe2+ were more than 50 % complete in 1.77 and 0.7 s, respectively. The reaction is an inner sphere electron transfer process as an outer sphere process is symmetry-forbidden. Studies show that Mn(III) intermediates are produced during the reaction. The fast kinetics makes this reaction significant to consider when modeling manganese oxide and reduced iron in environmental redox systems.