Photoluminescence, electronic absorption, and pH studies of a poly(ethylene-3,4-dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) dispersion as a function of the PEDOT:PSS concentration are shown to provide a better understanding of the effect of PSS on the intramolecular conductivity of; PEDOT chains. Particularly, concentration changes of PEDOT:PSS were found to be accompanied with different extents of dissociation of protons in the solution and different charge states of PSS chains, respectively, which affect the electrostatic interaction between PSS and PEDOT and intramolecular conductivity in the PEDOT backbone.