The electrochemical behavior of the [(AuCl4)-Cl-III](-)[(AuCl2)-Cl-I](-)-Au(0) redox system in room temperature ionic liquid (RTIL) of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) has been investigated quantitatively using an in situ electrochemical quartz crystal microbalance (EQCM) technique based on a Pt film-coated quartz crystal electrode (Pt-QCE). A series of two-electron (2e) and one-electron (le) reductions of the [(AuCl4)-Cl-III](-) to [(AuCl2)-Cl-I](-) and [(AuCl2)-Cl-I](-) to Au metal were recognized at the Pt surface. Besides, the disproportionation reaction of [(AuCl2)-Cl-I](-) (i.e., the 2e-reduction product of [(AuCl4)-Cl-III](-)) to [(AuCl4)-Cl-III](-) and Au metal was also observed. Electro-dissolution of the Au deposited on the Pt electrode through a le-oxidation reaction in the presence of chloride ions was also confirmed using the Pt-QCE based EQCM technique. A 2e-oxidation reaction of [(AuCl2)-Cl-I](-) (i.e., the dissolved product) to [(AuCl4)-Cl-III](-) along with the oxidation of Cl- ion on the Pt surface was also realized at high anodic potential. The results demonstrate that in situ EQCM technique is applicable and powerful in elucidating electrochemical surface phenomena accompanying a mass change in RTIL.