The decomposition of H(2)O(2) on iron minerals can generate •OH, a strong oxidant that can transform a wide range of contaminants. This reaction is critical to In Situ Chemical Oxidation (ISCO) processes used for soil and groundwater remediation, as well as advanced oxidation processes employed in waste treatment systems. The presence of dissolved silica at concentrations comparable to those encountered in natural waters decreases the reactivity of iron minerals toward H(2)O(2), because silica adsorbs onto the surface of iron minerals and alters catalytic sites. At circumneutral pH values, goethite, amorphous iron oxide, hematite, iron-coated sand, and montmorillonite that were pre-equilibrated with 0.05-1.5 mM SiO(2) were significantly less reactive toward H(2)O(2) decomposition than their original counterparts, with the H(2)O(2) loss rates inversely proportional to SiO(2) concentrations. In the goethite/H(2)O(2) system, the overall •OH yield, defined as the percentage of decomposed H(2)O(2) producing •OH, was almost halved in the presence of 1.5 mM SiO(2). Dissolved SiO(2) also slowed H(2)O(2) decomposition on manganese(IV) oxide. The presence of dissolved SiO(2) results in greater persistence of H(2)O(2) in groundwater and lower H(2)O(2) utilization efficiency and should be considered in the design of H(2)O(2)-based treatment systems.