<p>Floodplain sediments play a central role in redox-driven biogeochemical cycles through their electron exchange capacity, yet its origin remains unclear because of strong chemical heterogeneity. Here we quantified the electron-accepting and electron-donating capacities of 45 sediments from two Yangtze River floodplains using mediated electrochemical analysis. We identified that electron-accepting capacity was controlled by acid-extractable reactive ferric iron associated with iron (oxyhydr)oxides and expandable phyllosilicates. Electron-donating capacity was mainly contributed by terrigenous humic fractions (12.9–61.2%) and reactive ferrous iron (38.8–87.1%). Lignin-like compounds with low double bond equivalent were the dominant organic contributors to electron-donating capacity. For ferrous iron, its contribution to sediment electron-donating capacity was much smaller than predicted by chemometric analysis because a large fraction of ferrous iron associated with phyllosilicates was redox-inert due to chemical and structural constraints. The composition of electron exchange capacity dictated microbial selection of terminal electron acceptors, driving the conversion of high-redox-potential electron-accepting capacity into electron-donating capacity. This transformation of electron exchange capacity has important implications for suppressing methanogenesis in floodplains and for oxidant consumption during contaminated aquifer remediation.</p>