The influence of poly(lysine) and poly(arginine) on silicate solutions and silica sols has been investigated. In both cases, solid formation could be observed that appeared to depend on the polymer chain length. Polyelectrolytes induced gelation of diluted sodium silicate solutions. Studies of the silicic acid content using the colorimetric molybdosilicate method suggested that polymers act as gelating agents of silica oligomers via electrostatic interactions that favor condensation. In the case of silica sols, quasi-elastic light scattering measurements indicate that particles are first adsorbed on the polymer chain to form aggregates that then flocculate in the presence of additional peptides. Structural characterizations of the solids obtained in both cases were consistent with the proposed models. These results are discussed in the frame of biogenic silica formation at the protein/silica interface.