Chronocoulometry was employed to measure the total charge density of the Au(111) electrode in the presence of the Cu underpotential deposition reaction (UPD) from a bromide-containing electrolyte. The back-integration procedure and the thermodynamics of the perfectly polarized electrode were then used to calculate the Gibbs excess of copper coadsorbed with bromide. We found that copper and coadsorbed bromide form a bilayer in which copper adatoms are in contact with the gold surface and bromide ions form the top layer. We estimated the number of electrons flowing to the interface per one adsorbed copper atom. The number is equal to two in the whole range of potentials of the Cu UPD reaction. This result indicates that adsorption of Cu does not involve a partial charge-transfer mechanism.