The difference in ion uptake between EG/GOPS-PEDOT:PSS and crys-PEDOT:PSS was also reflected in the EQCM data (Figure ). EG/GOPS-PEDOT:PSS displayed mass uptake during dedoping to a low oxidation state (-0.7 V vs Ag/AgCl) and mass decrease when doping to a high oxidation state (+0.5 V vs Ag/AgCl) with respect to the equilibrium oxidation state (~0.1 V vs Ag/AgCl), consistent with solely cation transport. In comparison, crys-PEDOT:PSS shows a mass increase both upon dedoping and doping to the low (-0.7 V vs Ag/AgCl) and high oxidation states (+0.5 V vs Ag/AgCl), respectively, indicating a more complicated transport environment with both mobile anions and cations contributing. Normalizing the EQCM mass changes by film thickness it becomes apparent that the mass increase (+14%) accompanying dedoping to a low oxidation state (-0.7 V vs Ag/AgCl) and mass decrease (-11-12% ) accompanying redoping to the equilibrium oxidation state (~0.1 V vs Ag/AgCl), were nearly identical for both EG/GOPS-PEDOT:PSS and crys-PEDOT:PSS. When further doping to a high oxidation state (+0.5 V vs Ag/AgCl), crys-PEDOT:PSS displayed a 3.4% mass increase, compared with 8.3% mass decrease in EG/GOPS-PEDOT:PSS.