The underpotential deposition (upd) of Pb monolayers at Ag(111) thin-film electrodes in perchlorate solutions have been investigated as a function of the substrate defect structure. Ag(111) films were grown epitaxially on muscovite mica by thermal evaporation and annealed at 300 degrees C at 10(-6) Torr for periods between 0 and 12 h. X-ray diffraction and electrochemical scanning tunneling microscopy indicate that thermal annealing significantly increases the degree of film crystallinity and smoothness. A more ideal voltammetric response for the upd of Pb was obtained using Ag thin-film electrodes that were annealed for periods longer than 6 h. Specifically, the width of the upd voltammetric wave, as well as the splitting between the cathodic and anodic peaks, was found to decrease with an increase in annealing time. Surface roughness factors (defined as the ratio of the electrochemically-active area relative to the geometric area) for the Ag(111) films decreased from 1.15 for unannealed films to 0.99 +/- 0.03 for films after 12 h of annealing. The results indicate that previously reported voltammetric features observed for the upd of Pb monolayers in perchlorate solutions are associated with Pb deposition at Ag(111) surface defect sites.