Research was performed to gain insight into the heterogeneous reaction of Cr(VI) reduction by zero-valent iron, which is frequently used in the treatment of contaminated groundwater using permeable reactive barriers. An electron probe microanalyzer was used to clarify in detail relevant aspects of the reaction with consequences for the conception of interpretative kinetic models. Spherical particles of iron with controlled grain sizes were used after being subjected to a previous washing with diluted acid in order to remove oxidation products. These spheres were immersed in solutions of Cr(VI) in nonagitated flasks using different operating procedures. The iron particles were photographed so that the time evolution of the grain size distribution could be established. A sample of the iron balls after the reaction and samples of the raw material and the precipitates of the reaction products were analyzed by backscattering electron images and elemental mapping produced by wavelength-dispersive spectroscopy. The analysis of the spatial distribution of the concentrations in iron, oxygen, and chromium indicates that there are three distinct mechanisms for the reaction with different limiting steps.