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Introducing ethylene glycol (EG) side chains to a conjugated polymer backbone is a well-established synthetic strategy for designing organic mixed ion-electron conductors (OMIECs). However, the impact that film swelling has on mixed conduction properties has yet to be scoped, particularly for electron-transporting (n-type) OMIECs. Here, we investigate the effect of the length of branched EG chains on mixed charge transport of n-type OMIECs based on a naphthalene-1, 4, 5, 8-tetracarboxylic-diimide-bithiophene backbone. We use atomic force microscopy, grazing-incidence wide-angle X-ray scattering (GIWAXS), and scanning tunneling microscopy to establish the similarities between the common-backbone films in dry conditions. Electrochemical quartz crystal microbalance with dissipation monitori1ng (EQCM-D) and in situ GIWAXS measurements reveal stark changes in film swelling properties and microstructure during electrochemical doping, depending on the side chain length. We find that even in the loss of the crystallite content upon contact with the aqueous electrolyte, the films can effectively transport charges and that it is rather the high water content that harms the electronic interconnectivity within the OMIEC films. These results highlight the importance of controlling water uptake in the films to impede the charge transport in n-type electrochemical devices. This article is protected by copyright. All rights reserved