Across electrolyte concentration and pH, Br -anions were rigorously excluded to levels below the detection limit (<0.001 Br -per EDOT repeat unit) due to Donnan exclusion arising from the fixed anionic charge of the excess PSS polyelectrolyte. This empirically validates the assumptions no mobile anion contribution implicit to previous PEDOT:PSS EQCM studies. However, in previous works the cation concentration was not measured directly but calculated from the charging currents, thus any faradaic side reactions not contributing to hole carrier density undermined the accuracy of the derived cation concentration within the OMIEC film.