Poly(3,4-ethylenedioxythiophene)/gold (PEDOT/Au) nanocomposites were prepared by in-situ redox reaction using PEDOT in the neutral state and AuCl3 as an oxidant. AuCl3 had a sufficient power to oxidize the neutral PEDOT film while it reduced to form gold nanoparticles in the polymer matrix. It was observed that the rate of redox reaction was strongly influenced by the solvent. Finally, the formation and coalescence of gold nanoparticles was clearly demonstrated by SEM and XPS experiments, and a rapid drop in film resistance was also monitored during the redox reactions.