Evidence is given for the ability of ellipsometry in providing detailed information on the early stages of electrode modification by electronically conducting polymers. The analysis of the optical data, simultaneously recorded with the electrochemical signal, for the potentiostatic electrosynthesis of Poly(3,4-ethylenedioxythiophene) (PEDOT) showed that a very thin inner layer (approximate to 15 nm) is deposited prior to the formation of an homogeneous polymer film, in contrast to the previouly described growth of Poly(3-methylthiophene)(P-3MeT) which is preceded by oligomers formation and originates a compact inner layer (up to about 60 nm). In-situ ellipsometric data also enabled to detect the structural modifications of PEDOT films upon doping, revealing that a 80nm thickness polymer shrinks about 23% from conducting to insulating stages.