A quantitative combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) analysis of Pb underpotential deposition (Pb,,,) in aqueous 0.1 mol dm(-3) perchloric acid solution at polycrystalline gold was achieved by comparing the cyclic-voltammetric, -gravimrtric, and -beam deflection responses. The refractive index responses (PBD) were commensurate with the large variation in index of refraction of Pb(ClO4)(2) with change in concentration (30.5 cm(3) mol(-1)), demonstrating that the technique is remarkably sensitive to monitor the formation of submonolayers of Pb-upd. In this work the current and mass variation signals were projected upon the beam deflection profile by using the mathematical tool of quantitative temporal convolution. We show that the overall process of Pb,,, occurs in three distinct stages. The first stage consists of water ejection from gold. In the second stage, metal underpotential deposition is accompanied by OHad displacement followed, in the last step, by water formation. The coupled diffusion coefficient of Pb(ClO4), transferring in 0.1 mol dm(-3) HClO4 solution was evaluated to be 6.25 X 10(-6) cm(2) s(-1) (C) 2001 The Electrochemical Society.