Organic mixed ionic-electronic conductors (OMIECs) are π-conjugated soft materials with ionic or ionophilic functionalization that imparts free ion miscibility and transport. OMIEC's ability to both transport and couple ionic and electronic charge make them attractive active materials for next generation bioelectronics, energy storage, 5 electrochromic, and neuromorphic computing applications. There has been a recent growth in characterization work with the aim of unraveling OMIEC structure-property relationships in order to direct synthetic design of OMIECs and improve performance in targeted applications. These efforts are challenging as OMIECs in application relevant conditions are often swollen with ions and solvent, and the resulting electrolyte-swollen structure is sensitive to electrochemical potential and the composition of the surrounding media. While significant advances have improved the understanding of application-relevant structure and electronic transport, significantly less work has explored ionic/solvent composition and transport in OMIECs. This critically limits the development of OMIECs, since understanding the structure property relationships that dictate mixed transport require knowledge of OMIEC composition, especially the concentration and distribution of ionic species.