This work describes the studies of tin underpotential deposition on Pt electrodes in acid media by using cyclic voltammetry and electrochemical quartz crystal microbalance techniques. The voltammetric results were analyzed and it was verified the same values of anodic charge densities for the Sri in the solutions of perchloric acid with tin sulfate or chloride. The charge density value found (380 muC cm(-2)), in the potential region from 0.4 to 0.8 V, was attributed to the oxidation of Sn to Sn4+ (0.9 monolayer). The electrochemical quartz crystal microbalance results showed a mass-charge ratio of 16 g mol(-1) (43 ng cm(-2)) and 8 g mol-1 (50 ng cm(-2)) in the same potential region for tin oxidation in different electrolyte solutions. These relationships are due to the formation of Sn(OH)(4) and SnO2, respectively. The differences observed in relation to the formation of Sn(OH)4 in solutions containing SnSO4 and SnCl, were due to the presence of HSO4- in these solutions. In this manner the Off can adsorb on Sri in perchloric acid media with SnSO4. The tin hydroxide and oxide are reduced in the scanning to the potentials between 1.55 and 0.05 V. (C) 2003 Elsevier Science Ltd. All rights reserved.