Experiments were conducted to investigate the fundamentals of silica sorption onto preformed ferric hydroxide at pH 5.0-9.5 and silica concentrations of 0-200 mg/L as SiO2. At all pHs studied, sorption densities exceeding monolayer sorption were observed at silica levels typical of natural waters. Under some circumstances, sorption equaled or exceeded a monolayer while the particle zeta potential remained positive, a phenomenon that is completely inconsistent with available surface complexation models. To address this deficiency, an extended surface complexation model was formulated in which soluble dimeric silica (i.e., Si2O2(OH)5-) sorbs directly to iron surface sites. This new model fit sorption density data up to 0.40 mol SiO2/mol Fe and accurately predicted trends in zeta potential and observed H+ release during silica sorption to ferric hydroxide.