We demonstrate an interfacial electrochemical process for the preparation of composite film of poly(3,4-ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNTs) at a water/oil (w/o) interface. The 3,4-ethylenedioxythiophene (EDOT) monomer and lithium perchlorate electrolyte were supplied from the oil phase (dichloromethane) and water phase, respectively. Firstly, acid-oxidized carbon nanotubes were assembled at the w/o interface with the assistance of a trace dosage of cationic surfactant, forming a CNTs network. And then the CNTs network acted as a "working electrode" and "reinforcement bars" for the electropolymerization of PEDOT, resulting in the production of PEDOT@CNTs composite film. Higher CNTs areal density (mu g/mm(2) CNTs) caused a faster radial growth of PEDOT at the CNTs network, namely a faster growth of "apparent area" of the composite film. The current-time curves of the electropolymerization demonstrated that there was an optimal areal density of CNTs network (0.40 mu g/mm(2)) to achieve the largest amount of polymerized PEDOT. These observations suggested that the assembled CNTs network and the formed PEDOT@CNTs composite film not only functioned as an electrical conductor for the electrochemical process, but also affected the mass transfer of the reactants. The prepared PEDOT@CNTs composite film showed heterogeneous structures with a capacitance of 0.148 F/C. The pre-assembled nano-particles network involved liquid-liquid interfacial eletropolymerization may be developed for the preparation of a series of conductive polymer@nanoparticle composite film. (C) 2014 Elsevier Ltd. All rights reserved.