Macromolecular phenolics are among the most abundant organic molecules in nature, yet their biological activities are largely unresolved because of their structural complexity and because of an inability to probe their functionality experimentally. We developed thin film and electrochemical methodologies to probe the redox properties of melanin, lignin, and humic acid, three of the most abundant phenolic materials. We observed that all three phenolic matrixes possess redox activity and can be repeatedly switched between oxidized and reduced states. Furthermore, we observed that melanin possesses pro-oxidant activities exemplified by the uncatalyzed generation of reactive oxygen species (ROS) upon exposure to air; however, this pro-oxidant activity is observed only for melanin films that are poised in their reduced state. Conversely, melanin's antioxidant radical-scavenging activities are insensitive to its redox state. These results demonstrate that natural phenolic matrixes are not inert but rather serve as open-source redox media with significant potential for impacting redox signaling and redox biology.