An innovative approach for assessing reduction in soils was recently introduced by Jenkinson (2003.) Indicator of Reduction in Soils (IRIS) tubes are manufactured by coating PVC pipe with a paint prepared from a ferrihydrite suspension. When the coated IRIS tubes are inserted into the soil, microbially mediated reduction causes dissolution of the iron oxide coating from the IRIS tubes which can be visually observed and easily documented. When the iron oxide is synthesized by adding KOH to a FeCl₃ solution, the mineral phase is essentially pure ferrihydrite, but paint prepared from pure ferrihydrite shows poor adhesion and durability qualities when applied to the PVC. Over time, and under particular conditions, the ferrihydrite is transformed to more crystalline forms such as goethite and hematite. By modifying the pH of the suspension when synthesizing the iron oxides, one may control the rate at which these mineral transformations occur. The effect of pH during synthesis and the effect of aging on the formation of iron oxide phases will be discussed as it pertains to adhesion and durability of iron oxide paint used in preparation of IRIS tubes for field use.