Ion transfer during ionomer contact electrification: Binding affinity controls charging Contact electrification occurs ubiquitously in nature, but the identity of charge carriers in most situations remains uncertain, with electrons, ions, or nanoscopic material fragments as viable candidates. One material where the species transferred seems more certain is ionomers, i.e., polymers that contain mobile ions balanced by fixed counter‐charges. When ionomers touch a neutral surface, the latter becomes charged in the sign of the mobile ion, strongly suggesting ion transfer. However, the mechanism and governing factors of transfer remain poorly understood. Here, we demonstrate that binding affinity between mobile ion and ionomer controls charge transfer with ionomers. We use ionomers with fixed anions and cations and perform ion exchange to create samples with a series of transferrable ions. We observe a strong binding‐affinity dependence for the anionic ionomer, such that mobile cations with the highest affinity transfers the least charge. Weaker, yet clear dependence was measured for the cationic ionomer, which follows the hydration free energy of the mobile anion. Using inductively coupled plasma optical emission spectroscopy (ICP‐OES), we confirm transfer of the mobile ions to the counter sample's surface. Our results confirm the identify and mechanism of charge transfer with ionomeric materials, with potential implications for contact electrification more broadly. Wiley 1