Nature Materials, Published online: 25 February 2026; doi:10.1038/s41563-026-02490-0Unlike common organic mixed ionic–electronic conductors that swell upon electrochemical doping, poly(benzimidazobenzophenanthroline) exhibits an unexpected cation-dependent deswelling at high doping levels. Operando measurements and modelling show that protic cations drive this response by expelling water and localizing charge through strong interactions with the polymer backbone, revealing a distinct deswelling mechanism.