T1 underpotentially deposited (upd) on Au(lll) has been studied using potential-step chronocoulometry and electrochemical quartz crystal microbalance to determine the actual coverage of T1. At anodic potentials, T1 adsorbs to the surface at a coverage that slowly increases up to 0.55 ML. At more cathodic potentials, the coverage rises sharply to a plateau of 0.79 ML where the surface is covered with a close-packed T1 monolayer. The electrosorption valency calculated based on this coverage data indicates that the low coverage overlayer is incompletely discharged but that the overlayer undergoes further discharge during the voltammetric features associated with complete monolayer formation. Voltammetric experiments also indicate that the presence of hydroxide anions plays a role in stabilizing the charged low coverage phase. The implications of this structural and charge information for the catalytic activity of the T1 upd system are discussed.