Through incorporation of SiO2 nanoparticles (NPs) into poly(3,4-ethylenedioxythiophene) (PEDOT) films, the physical volume effect of nanoparticles on the electrochromic properties of PEDOT was investigated. PEDOT-SiO2 nanocomposite thin films were prepared using SiO2 NPs, 3,4-ethylenedioxythiophene (EDOT), iron p-toluenesulfonate and an imidazole mixture. During incorporation, SiO2 NPs agglomerated to form a cluster, which affected the surface roughness of PEDOT films. Photoemission spectroscopic results of C 1s and S 2p show that incorporated SiO2 NPs did not chemically interact but heavily affect the conjugation length and packing density of PEDOT due to a physical volume effect. This physical effect of SiO2 NPs resulted in enhanced electrical conductivity of PEDOT and induced a blue shift of the maximum absorption peak in ultra violet-visible spectroscopic analysis. Also, a gradual enhancement in the electrochromic properties of PEDOT was obtained by increasing the amount of SiO2 NPs; in particular, the switching speed of PEDOT reduced from 3.34 s to 1.28 s with incorporation of 10 wt % SiO2 NPs. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 3080-3085, 2011