Polythiophene−magnetite composite layers have been prepared through the electropolymerization of 3-thiophene-acetic-acid in the presence of Fe3O4 nanoparticles in nitrobenzene. Stabilization of magnetite in this organic medium could be achieved by the reaction between surface −OH groups of the nanoparticles and the −COOH function of the monomers. Fourier transformed infrared spectroscopic (FT-IR) measurements evidenced the chemisorption of the monomer on the surface of the nanoparticles. By modifying the amount of iron-oxide in the polymerization solution, the inorganic material content of the layer could be increased up to 80 m/m%. Electrochemical results, including data obtained by electrochemical quartz crystal microbalance (EQCM), proved that the presence of Fe3O4 did not influence the redox properties of the polymeric film. In the presence of magnetite, an extraordinary microstructure can be detected, where the self-assembling magnetic component strongly determines the morphology of the composite, le...