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Theelectrical properties of eumelanin, a ubiquitous natural pigment,have fascinated scientists since the late 1960s. For several decades,the hydration-dependent electrical properties of eumelanin have mainlybeen interpreted within the amorphous semiconductor model. Recentworks undermined this paradigm. Here we study protonic and electroniccharge carrier transport in hydrated eumelanin in thin film form.Thin films are ideal candidates for these studies since they are readilyaccessible to chemical and morphological characterization and potentiallyamenable to device applications. Current–voltage (I-V) measurements, transient current measurementswith proton-transparent electrodes, and electrochemical impedancespectroscopy (EIS) measurements are reported and correlated with theresults of the chemical characterization of the films, performed byX-ray photoelectron spectroscopy. We show that the electrical responseof hydrated eumelanin films is dominated by ionic conduction (10–4–10–3S cm–1), largely attributable to protons, and electrochemical processes.To propose an explanation for the electrical response of hydratedeumelanin films as observed by EIS and I-V, we considered the interplay of proton migration, redoxprocesses, and electronic transport. These new insights improve thecurrent understanding of the charge carrier transport properties ofeumelanin opening the possibility to assess the potential of eumelaninfor organic bioelectronic applications, e.g. protonic devices andimplantable electrodes, and to advance the knowledge on the functionsof eumelanin in biological systems. [ABSTRACT FROM AUTHOR]