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Coupled ionic and electronic transport underpins processes as diverse as electrochemical energy conversion, biological signaling, and soft adaptive electronics. Yet, how chemical environments such as pH modulate this coupling at the molecular scale remains poorly understood. Here, we show that the protonation state of carboxylated polythiophenes provides precise chemical control over ion dynamics, doping efficiency, solvent uptake and mechanical response. Using a suite of multimodal operando techniques, supported by simulations, we reveal that pH dictates the balance of cation/anion uptake during electrochemical doping. Mapping across pH uncovers a quasi–non-swelling regime (≈pH 3–3.5) where charge compensation proceeds with minimal volumetric change yet pronounced stiffening. These findings establish molecular acidity as a general strategy to program ionic preference and mechanical stability, offering design principles for pH-responsive mixed conductors and soft electronic materials that couple ionic, electronic, and mechanical functionality. 

Organic mixed ionic–electronic conductors (OMIECs) are an emerging class of polymeric materials with opportunities for applications in bioelectronics, neuromorphic computing, and various sensing technologies owing to their mixed conduction characteristics. The performance and long‐term operational stability of OMIECs, particularly in aqueous environments, c an be influenced by the dynamic interactions between polymer functionalities and electrolyte species. This mini review highlights the necessity of integrating advanced operando characterization techniques and computational modeling to successfully investigate structure–property relationships. Then, recent progress in understanding how sidechain design dictates ion transport, hydration, swelling behavior, and mixed conduction properties is summarized. Furthermore, the significant impacts of electrolyte composition on doping mechanisms, structural stability, and device performance are explored; and the persistent challenges associated with extensively studied ethylene glycol sidechain designs and emerging hybrid sidechain strategies that incorporate ionic moieties are examined. Recognizing the current limitations in understanding these complex systems, particularly regarding long‐term stability, this outlook focuses on elucidating fundamental structure–property relationships and degradation mechanisms. This understanding is crucial for the rational design and future development of robust and high‐performance OMIEC materials for organic electrochemical transistor applications.