The electropolymerization of 3-methylthiophene (3-MeTh) in LiClO4 + CH3CN is monitored by in situ probe beam deflection (Mirage effect). Analysis of current and beam deviation dependences with time at constant potential (chronoamperometry and chronodeflectometry) is made by using a new treatment based on parallel convolutions with transfer functions specific to each species moving between the electrode and the solution. Considered species are monomers arriving at the electrode, oligomers and protons produced by the polymerization reaction and anions coming from the electrolyte for balancing the oxidation of the polymer. The mechanism of P(3-MeTh) formation is confirmed and quantitative determination of reaction and transport properties is made, allowing an estimation of the proportion of oligomers undertaking deposition onto the electrode with respect to oligomers released towards the solution. (C) 2004 Elsevier B.V. All rights reserved.