Water-soluble conducting poly(2-(3thienyloxy)ethanesulfonic acid) (PTOESA)/V2O5 nanocomposite, (PTOESA)xV 2O5, was prepared by simply mixing PTOESA with V 2O5 wet gel at room temperature. XRD data showed that the interlayer spacings of (PTOESA)xV2O5 films are 14.0±1.5Å and increased as the polymer content increased. These values are consistent with the insertion of polythiophene chains into the V 2O5 layer gallery. The formation of alternative layers of PTOESA and V2O5 was further supported by depth profile SIMS analyses. Cyclic voltammograms of (PTOESA)xV2O 5 film showed two pairs of redox peaks with colors varying from orange, yellowish green, green, to purple blue, depending on the stoichiometry of the nanocomposites. Moreover, a synergetic effect was observed on the electrochromic properties of these nanocomposites. It was found that the optical contrast (ΔOD) of the composites is better than those of PTOESA and V2O5 at the film thickness from 150 to 500nm. The oxidation optical response time of (PTOESA)xV2O 5 is independent of the stoichiometry and falls in between those of PTOESA and V2O5. At higher polymer content (x>0.5), the reduction optical response time of (PTOESA)xV2O 5 is smaller than those of PTOESA and V2O5. Variable temperature conductivity data showed that the conductivity of (PTOESA)xV2O5 films increased as temperature increased, characteristic of thermal activated behavior, which was dominated by the interparticle contact resistance. The room-temperature conductivity of water-soluble (PTOESA)xV2O5 films was in between those of PTOESA and V2O5 xerogel and higher conductivity was found in the composite with lower polymer content. The anomalous conductivity of (PTOESA)xV2O5 with high PTOESA content may be due to the reason that the higher the polymer content, the bigger the grain size of (PTOESA)xV2O 5 film as revealed with scanning electron microscopy and AFM micrographs.