Redox reactions involving soil minerals and materials are important processes in environmental chemistry, but unfortunately they only have been characterized in the solution phase. The lack of a suitable method has prevented investigations of the mineral surface component of redox reactions. The authors present a model system which ultraviolet-visible spectroscopy is employed to observe a surface redox reaction. A synthetic MnOx is reacted with Cr(III), resulting in Cr oxidation to the hexavalent state and the reduction of the MnOx mineral. A scattered transmission accessory allowed the authors to observe spectroscopically both the appearance of Cr(VI) and the attenuation of MnOx in a suspension. Loss of absorbance intensity by MnOx suspensions may be useful as a general method to follow redox reactions as a function of the solid phase rather than relative to a specific reactant. This spectroscopic approach provides the capability to monitor in situ reactions of solid-phase components and is useful for examining transformations of metallic and organic wastes and naturally occurring materials in the environment.