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Highly conductive films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) were prepared by casting the PEDOT/PSS aqueous dispersion of colloidal particles containing ethylene glycol (EG). By addition of 3% of EG, the electrical conductivity was significantly increased from 3 to 430 S/cm. The EG was crucially important for (i) removal of the insulating PSS from the surface of the PEDOT/PSS particles, (ii) crystallization of PEDOT molecules, and (iii) aggregation of the particles, which improved both intra- and inter-particle transfer of charge carriers. A further increase of the EG concentration, however, decreased the conductivity, which was due to the morphological inhomogeneity by the phase segregation.