Cd underpotentially deposited on Au(lll) has been studied using potential-step chronocoulometry and electrochemical quartz crystal microbalance investigations to determine the actual surface coverage of Cd. In the potential region where Cd undergoes underpotential deposition (upd), the coverage is observed by chronocoulometry to rise to a limiting value of 0.66 ML. The electrosorption valency calculated on the basis of this coverage is 0.5, indicating that the Cd adatoms are not fully discharged on the surface. Potential-step chronocoulometry has been used at various sulfate concentrations to determine the surface coverage of sulfate anion in the presence of underpotentially deposited Cd. The coverage is negligible in the absence of underpotentially deposited Cd at the point of zero charge. It rises to a value of 0.23 ML in the Cd upd region, indicating that sulfate is specifically adsorbed with underpotentially deposited Cd at negative electrode potentials. Quartz crystal microbalance measurements indicate that there is considerably larger mass change at the surface during Cd upd than that expected on the basis of the measured coverages of Cd and sulfate. The extra mass is explained in terms of additional sulfate required in the outer Helmholtz plane to balance the residual positive charge on the underpotentially deposited Cd atoms.