Environmentally compliant organic–inorganic hybrid coatings for efficient corrosion protection of metallic surfaces are potential alternatives to the current method based on chromate passivation. In this context PMMA–siloxane–silica (PMMA–SS) hybrid films were prepared using the sol–gel process from the radical copolymerization of methyl methacrylate and 3-(trimethoxysilyl)propyl methacrylate followed by acidic hydrolysis and polycondensation of tetraethoxysilane (TEOS), under variation of the ethanol to H₂O ratio (0.0–1.0). The structural properties of about 2 μm thick coatings, deposited by dip-coating onto carbon steel, were related with their corrosion protection efficiency. The correlation of data obtained by X-ray photoelectron spectroscopy, nuclear magnetic resonance and small angle X-ray scattering has shown for intermediate ethanol to H₂O ratios the highest connectivity (∼83%) of the inorganic phase, bonded covalently to organic moieties, yielding a dense and homogeneous nanocomposite structure with high thermal stability, very good adhesion to the metallic substrate and excellent barrier properties. The electrochemical impedance spectroscopy measurements have shown for coatings prepared at intermediate EtOH/H₂O ratios a high corrosion resistance of almost 10 GΩ cm², which remained unchanged for more than 6 months in contact with 3.5% NaCl solution and more than 3 months exposed to an acidic NaCl environment.