Underpotential deposition (upd) and adsorption of Bi species have been studied at Pt electrodes in perchloric acid solutions using the electrochemical quartz crystal microbalance (EQCM). Mass transients reveal extensive adsorption of cationic Bi species at potentials positive of the upd region (where the electrode surface is oxidized) in 0.1 M HClO4. The bulk of this adsorption seems to derive from hydrolyzed Bi(III) species rather than Bi3+ since it is strongly dependent on pH, being much reduced when the acid concentration is increased to 1.0 M where hydrolysis is largely absent. Increasing the pH leads to an increase in adsorption. The extent of adsorption of cations prior to development of the upd deposit in 0.1 M HClO4 leads to mass changes that are smaller than expected when the potential is scanned into the upd region, since the deposit is formed largely from species already adsorbed at the electrode surface. The mass response also reveals an arrest on the positive-going scan that is suggested to be the result of a change in the nature of Bi at the surface from Bi to BiOH2+ or Bi(OH)2+. When a Pt electrode with adsorbed Bi is examined in a 0.1 M HClO4 electrolyte free of Bi(III), a mass increase is seen to be associated with the transition between upd Bi and adsorbed Bi(III) and this is attributed to the fact that the adsorbed Bi(III) is largely present as hydrolyzed species.