Linear poly(ethylene imine) (PEI) can self-organize into fibrous aggregates with a crystalline core and a brushlike shell of ethyleneimine (EI) segments. Silicification from alkoxysilane mediated by PEI aggregates easily produces silica nanofibers (20–23 nm in diameter) with a core of axial, crystalline PEI filaments (5–7 nm in width) and a shell of silica (6–8 nm in thickness). Removing the axial filament of PEI from the silica nanofiber by calcination produces silica nanotubes. More interestingly, a nanowire-like platinum lining is formed in the silica nanofiber—a result of the PEI filament's ability to reduce PtCl42–. The degree of polycondensation, composition, and surface area of the resulting silica are investigated by 29Si magic angle spinning NMR spectroscopy, elemental analysis, and N2/He adsorption–desorption measurements. The time course of the reaction and the amount of silica source needed for silica-fiber formation confirm that silica deposition occurs exclusively and site selectively on the surface of the organized, fibrous PEI. Thus, the fibrous aggregates of PEI appear to be highly suitable for depositing silica fibers from both tetramethoxysilane and tetraethoxysilane.