The piezoelectric detection of change in surface energy combined with the use of electrochemical quartz crystal microbalance (EQCM) was applied to underpotential deposition (upd) of silver on gold electrode. The upd of silver on gold was performed with a potentiodynamic method (10 mV s-1) in a deaerated solution of 0.1 mol dm-3 Na2SO4 + 10(-3) mol dm-3 H2SO4 + 5 x 10(-5) mol dm-3 Ag2SO4. The amplitude, Absolute value of A and phase angle, phi, of the piezoelectric signals changed depending on the adsorbed amount of silver in the potential range of 1.10-0.56 V (she). Particularly, two characteristic potentials at which Absolute value of A took a minimum value and phi changed by about 140-degrees, appeared in both anodic and cathodic potential sweep. The characteristic potentials, E1 = 0.62 V, and E, = 0.90 V (she) correspond to the potentials of electrocapillary minimum and maximum, respectively. The presence of two characteristic potentials during the upd was explained by taking into account the changes in adsorption state of HSO4- or SO42- ions. The EQCM results indicated that the complete charge transfer of silver ions, z = 1 takes place during the upd. The piezoelectric detection of change in surface energy responded sensitively to the structural changes in the upd adsorbate as compared with the EQCM.