Poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) was synthesized by oxidative polymerization and pH dependence of structure and electrical conductivity of the cast films were investigated by means of UV-vis-NIR, XRD, AFM analyses, and four-point technique. It was found that the neutralization of PEDOT/PSS with NaOH decreased the absorption of bipolarons representing the PEDOT in the highly doped state and disrupted the π-π stacking of the PEDOT crystalline structure, which lowered the electrical conductivity by six orders of magnitude with increasing the pH from 1.7 to 13.