The electropolymerization of 3,4-ethylenedioxythiophene (EDOTh) on platinum, under both potentiostatic and potentiodynamic control, has been analysed by electrochemical and ellipsometric measurements, simultaneously recorded. The experimental procedure for the acquisition of ellipsometric data, developed to follow the electrode surface modification along the whole process, allowed to collect information on the evolution of the polymer film properties with time and also as function of the electrode potential. A two-homogeneous phase model has been successfully used to describe the ellipsometric experimental parameters enabling to obtain from the azimuth angle Psi and phase shift Delta, simultaneously the thickness and the complex refractive index of poly(3,4-ethylenedioxythiophene) (PEDOTh) films at different growth stages and at various doping levels. Since both electropolymerization methods originate two-layered films, the calculated optical parameters (for the inner and outer layers) are an adequate basis of analysis which have been used in the interpretation of the structural modifications occurring during the growth of PEDOTh films (up to a thickness of about 200 nm) as well as of those induced by the electrode potential, during and after the redox conversion of polymer films with different thicknesses. (C) 2010 Elsevier B.V. All rights reserved.