  Electrolyte gating using ionic liquid electrolytes has recently generated
considerable interest as a method to achieve large carrier density modulations
in a variety of materials. In noble metal thin films, electrolyte gating
results in large changes in sheet resistance. The widely accepted mechanism for
these changes is the formation of an electric double layer with a charged layer
of ions in the liquid and accumulation or depletion of carriers in the thin
film. We report here a different mechanism. In particular, we show using x-ray
absorption near edge structure (XANES) that the previously reported large
conductance modulation in gold films is due to reversible oxidation and
reduction of the surface rather than the charging of an electric double layer.
We show that the double layer capacitance accounts for less than 10\% of the
observed change in transport properties. These results represent a significant
step towards understanding the mechanisms involved in electrolyte gating.
