The hydrolysis of Fe-Si systems with Si/Fe ratios between 0 and 4 leads to the formation of poorly crystalline or, more frequently, of long-range disorganized precipitates. The increase of Si/Fe molar ratios results in an dramatic change of Fe polymerization. The formation of double and single corner-sharing Fe linkages is reduced compared to pure Fe hydrolysis products. The growth regime depends on the Si concentration in the system. Three-dimensional and two-dimensional growth of Fe colloids occurs at low and high Si/Fe ratios, respectively, systems with Si/Fe ratios around 1 representing a crossover between these two regimes. Though Si neighbors cannot be detected unequivocally by Fe K-edge EXAFS, their presence in the close environment of Fe atoms is evident from the change in Fe speciation.