The nature of chemical mechanisms, bondings, and structures in a single-step polyestermelamine enamel/H 3 PO 4 systems, in particular, at the interface of polymer/phosphate/steel, was investigated by using ac impedance analysis and 80 o grazing angle reflectance Fourier transform infrared spectroscopy. An in-situ phosphatizing coating on «Q» panel was illustrated and show to behave like a pure capacitance. The «simultaneous» chemical reaction of H 3 PO 4 wth «Q» panel surface and polymer resin was evidenced to produce a corrosion protective barrier by the foration of interfacial layer of crystalline ion phosphate and to enhance coating adhesion by the covalent bondings of P-O-C linkage.