In summary, we quantified the ionic composition of different species of p-type OMIEC after electrolyte exposure and electrochemical cycling. Among them, the presence of polyelectrolyte in OMIEC significantly affects the ionic species involved in the electrochemical cycle. The polyelectrolyte functioned as a buffer and minimized the effect of electrolyte concentration and pH on the changes in OMIEC composition. Specifically, the negatively charged polyelectrolyte PSS -prevents mobile anions from entering the polymer due to Donnan exclusion. The passive ion uptake under electrolyte exposure in PEDOT:PSS with different polyelectrolyte contents was simulated by an appropriate Donnan model. The amount of fixed charge controls the strength of Donnan exclusion and thus controls the ultimate film composition. We also analyzed the mesoscale structure and ion distribution using GISAXS, which shows mild ion segregation in the polymer blend EG/GOPS-PEDOT:PSS and little ion segregation in single-component OMIEC pg2T-TT during the electrochemical cycling. In the crys-PEDOT:PSS with a well-defined fibrillar crystalline structure, the doped state of the polymer shows a significant inhomogeneous ion distribution and a contraction of the inter-fibrillar void/domain.