Silicates can greatly affect the characteristics of iron oxidation products, and thereby floc formation, in water treatment. This study investigated the effects of silicate on iron oxidation in six groundwaters from the west coast of Finland. The waters contained 13-29 mg SiO(2)/L, were soft and acidic, and had high contents of iron, manganese, and organic matter. Sample pH was adjusted to 6.9 or 7.9, and a known concentration, 0-80 mg SiO(2)/L, of silicate was added. After oxidation with hydrogen peroxide, the sample was filtered to determine the iron size fractions. The results showed that even small amounts of added silicon decreased the iron particle size. The critical silicon:iron molar ratio was 1.3-2.5, above which fine iron particles, <0.1 μm, were formed. Increasing pH weakened the effect of silicon. Addition of silicon did not affect iron that was already in oxidized form.