Poly(3,4-ethylene dioxythiophene) (PEDOT) is now the most common conductive polymer chosen for functional applications because it is readily available as a shelf-stable aqueous dispersion doped with poly(styrene sulfonic acid) (PSS), and cast films of PEDOT are both electronically conductive and optically transparent. Despite the widespread use of PEDOT:PSS, little is known about the structure of this polyelectrolyte complex in aqueous dispersion and in cast films. It has been reported that the electronic properties of these films can be manipulated through addition of aprotic solvents to the aqueous dispersion to increase the conductivity by orders of magnitude (so-called ‘secondary' doping). By using perdeuterated PSS (dPSS) in PEDOT:PSS dispersion, small angle neutron scattering (SANS) measurements can provide information about the structure of PEDOT:PSS in both aqueous dispersions and in thin films. For the aqueous dispersions, the SANS data show two distinct length scales. The scattering at lower angles arise from aggregates of the PEDOT:dPSS complex and the scattering at higher angles arise from free dPSS chains. When dimethylsulfoxide (DMSO) is added to PEDOT:dPSS solution, no changes are observed in the SANS data. For PEDOT:dPSS solid films, the lower angle scattering intensity appears to follow a -3 power law suggesting a morphology with diffuse interfaces. For solid films of PEDOT:dPSS with DMSO cast onto silicon wafers, the intensity of the lower angle scattering changes with increasing the DMSO concentration. However, no changes in the scattering are observed from solid films cast from PEDOT:dPSS with increasing concentrations of ethylene glycol. These measurements can provide information about changes in the film structure with aprotic solvent doping and may provide the physical insight needed to further improve or optimize property-structure relationships for PEDOT:PSS.