In voltage-dependent ion channels, pore opening is initiated by electrically driven movements of charged residues, and this movement generates a gating current. To examine structural rearrangements in the Shaker K⁺ channel, basic residues R365 and R368 in the S4 segment were replaced with histidine, and gating currents were recorded. Changes in gating charge displacement with solvent pH reveal voltage-dependent changes in exposure of the histidine to solvent protons. This technique directly monitors accessibility changes during gating, probes the environment even in confined locations, and introduces minimal interference of gating charge motion. The results indicate that charges 365 and 368 traverse the entire electric field during gating. The remarkable implication of the successive exposure of histidine to each side of the membrane is that in a pH gradient, the voltage sensor transports protons.