Considering crys-PEDOT:PSS ion concentration changes alone, upon doping to a high oxidation state (large anion concentration increase, moderate cation concentration decrease), a mass increase is expected (opposite to EG/GOPS-PEDOT:PSS, large cation decreases only) and is observed with EQCM. Though considering crys-PEDOT:PSS ion concentration changes alone upon dedoping to a low oxidation state (large anion concentration decrease, moderate cation concentration increase), a slight mass decrease would be expected. Instead, a moderate mass increase was observed indicating enhanced water uptake accounting for this discrepancy. Thus, the amount of water accompanying ion transport cannot be presumed to be constant across oxidation state. In sulfuric acid treated crys-PEDOT:PSS, the strength of acid during crystallization allows the control of the excess PSS -concentration, i.e. allows the tuning of fixed cationic charge within the OMIEC. Leveraging this, we investigated the effect of fixed charge upon ion uptake with XPS compositional analysis on electrolyte exposed PEDOT:PSS previously treated with sulfuric acid of different concentration (Figure ). XPS allowed the simultaneous measure of the PSS:PEDOT ratio and the qualitative ion concentration changes, while XRF was carried out to confirm the ion concentration trends through the bulk.