Changes in surface mass at a Au electrode during cyclic voltammetry in acidic media were monitored with an electrochemical quartz-crystal microbalance. An unexpectedly large increase in surface mass was observed during the positive potential scan in the pre-oxide region beginning ca. 0.5 V negative of the potential for onset of formation of surface oxide (AuO). This mass increase is virtually independent of the acid and is interpreted on the basis of adsorption of H2O via hydrogen bonding to the submonolayer of hydrous oxide (AuOH) generated in the pre-oxide region. The mass increase is consistent with an increase in surface hydration by about 32 H2O molecules per AuOH site.