Having quantified the ion composition of prototypical polyelectrolyte:conjugated polymer and conjugated polymer electrolyte OMIECs, we turned to the top performing OMIEC, sulfuric acid crystallized PEDOT:PSS (crys-PEDOT:PSS). Post treatment of PEDOT:PSS films with concentrated sulfuric acid removes excess PSS and drives the nanofibrillar crystallization of the remaining PEDOT:PSS. Treatment with 95% sulfuric acid reduces the PSS-to-PEDOT ratio We have previously reported the XRF determined composition of 95% sulfuric acid crys-PEDOT:PSS in the doped and dedoped states (Figure ). Quantitative comparison of crys-PEDOT composition with EG/GOPS-PEDOT:PSS and pg2T-TT are complicated due to substrate effects (thickness dependent Si absorption of S fluorescence leading to likely underestimate of relative Rb and Br concentrations). Qualitatively, crys-PEDOT:PSS exposed, doped, and dedoped composition is radically different from EG/GOPS-PEDOT:PSS as the removal of excess PSS during acid treatment suppresses the effect of Donnan exclusion (Figure ). Thus, both cations and anions are free to transport in and out the film, and resulting Br - concentrations are on the same order as Rb + concentrations.