Reactions mediated by iron mineral surfaces play an important role in the fate of organic contaminants in both natural and engineered systems. As such reactions proceed, the size, morphology, and even the phase of iron oxide minerals can change, leading to altered reactivity. The reductive degradation of 4-chloronitrobenzene and trichloronitromethane by Fe(II) associated with goethite (α-FeOOH) was examined by performing sequential-spike batch experiments. The particle size and size distribution of the pre- and postreaction particles were quantified using transmission electron microscopy (TEM). Results demonstrate that the degradation reactions result in goethite growth in the c-direction. Furthermore, pseudo-first-order reaction rate constants for the degradation of 4-chloronitrobenzene and trichloronitromethane and for the loss of aqueous Fe(II) decrease dramatically with each subsequent injection of organic compound and Fe(II). This result indicates that the newly formed material, which TEM and X-ray diffraction results confirm is goethite, is progressively less reactive than the original goethite. These results represent an important step toward elucidating the link between mineral surface changes and the evolving kinetics of contaminant degradation at the mineral-water interface.