Two-pore domain (K2P) potassium channels are important regulators of cellular electrical excitability integrating a wide range of signalling pathways involving phosphorylation, various lipids, temperature, intracellular/extracellular pH and activation by membrane depolarisation.We have previously reported that in TREK-1 that the primary gating mechanism reside close to, or within the selectivity filter and does not involve gating at the cytoplasmic bundle crossing as seen in e.g. voltage activated K+ or Kir channels. Here investigated the ion dependence of gating in K2P channels in more detail. We found that in TREK-1 and TRAAK channels voltage activation dominated the strongly outward rectifying current-voltage (I-V) relationships in these channels and critically depended on the ion species in the intracellular pore. TREK-1 channels bathed in intracellular Na+ exhibited no voltage activation upon depolarisation, with K+ moderate activation was seen while in Rb+ and Cs+ strongly voltage activated TREK-1 currents were observed. In TRAAK channels this ion activation appears to be the primary mechanism underlying outward rectification as here even with intracellular K+ large voltage activated current were observed that resembled Kv channels currents. Mutations at the selectivity filter markedly altered the ion dependence of voltage gating in TREK-1 and TRAAK channels. These findings suggest that voltage activation arises from the voltage depend interaction of the permeant ion with the selectivity filter and, accordingly, the permeant ion appears to represent the gating charge and the selectivity filter the voltage sensing structure in K2P channels. This represents a novel concept of voltage sensing in K+ channels.