The known corrosion protection principles of intrinsically conducting polymer (ICP) coatings involve complex interactions with the metallic surface. A set of electrochemical substrate pretreatments were developed in a previous work to modify the properties of the metal/ICP interface in a controlled way, leaving the ICP coating properties unchanged. The impedance of the pretreatment layers prove to have a strong impact on the corrosion protection performance of ICP systems. To have a better understanding of that impact, in situ visual spectroscopic ellipsometry (Vis-SE) and Raman spectroscopy were combined in the present study to successfully determine the composition and the thickness of these pretreatment films. The in situ results prove to be reliable because of the absence of air exposure for the highly sensitive iron and steel substrates. The three substrate pretreatments (named galstat, 1-step, and 2-step) discussed in this paper generate an interfacial layer between the metallic substrate and the ICP coating, with a variable magnetite and iron oxalate content and a thickness ranging from about 50 nm to 1 μm.