A cyclic voltammetric study of the behaviour of Br- and Br-3(-) at Pt electrodes, in the potential range between hydrogen and oxygen evolution, is described. Different experiments were carried out, in the presence of Br- alone, and with a mixture of Br- and Br-3(-), in which the ratio between the species has been kept constant and equal to 1. The halide concentration was varied between 4 x 10(-6) and 1 x 10(-3) mol dm(-3), at constant ionic strength, in 1 M HClO4 as well as in 1 M NaClO4 adjusted to a pH of 2. Underpotential deposition of Br is observed at potentials as low as -0.125 V vs SCE. The adsorption parameters of Br species were determined from the adsorption/desorption peak pair in the hydrogen adsorption/desorption region, and from the oxide reduction peak data. In the absence of oxygen adsorption, a relatively high coverage of the electrode surface is attained. A Langmuir-type adsorption is observed under the different experimental conditions.