The water dispersions of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid) (PEDOT:PSS) as colloidal particles were synthesized by oxidative polymerization with different composition ratios of repeating units for PSS and PEDOT (alpha = 1.4-8.3). The role and effect of the PSS on hierarchical structure and electrical conductivity of the PEDOT: PSS were investigated systematically by means of X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and conductive atomic force microscopy (c-AFM). It was found that the PEDOT: PSS colloidal particles stably dispersed in water at alpha >= 2.3, while small primary particles aggregated to form large secondary particles in water at alpha = 1.4 because the zeta potential dropped owing to the less PSS. The PEDOT: PSS showed paracrystalline structure where highly conductive PEDOT nanocrystals uniformly distributed in the less conductive PSS matrices. The electrical conductivity was strongly dependent on the composition ratio and attained as high as 700 S/cm at alpha = 2.3 where a positive correlation was seen between the electrical conductivity and number of conductive particles favorable for hopping of charge carriers in between the nanocrystals. (C) 2018 Elsevier Ltd. All rights reserved.