Strongly adsorbed bromine monolayers are formed when Pt(111) electrodes are exposed to aqueous bromide solutions at open circuit or to bromine gas atmospheres. These adlayers are stable in air as well as in bromide-free acidic electrolyte (at sufficiently high potentials). The bromine coverage for these adlayers (evaluated from charge displacement experiments during potentiostatic CO adsorption) is very similar for these two dosing procedures and close to that existing at the Pt(111)/aqueous bromide electrolyte interface in a relatively wide potential range positive to the voltammetric peaks. This surface coverage is around 0.46 Br/Pt. These irreversibly adsorbed Pt(111)-Br adlayers are stable enough for protecting the surface against contamination for prolonged periods of time, providing a surface protection procedure simpler than the widely used I-CO replacement method. The structure of the bromine adlayer can be studied ex situ by using STM, due to the immobile character of the adlattice. The existence of a densely, close-packed hexagonal adlayer of bromine atoms has been evidenced. This adlayer gives rise to different coincidence structures in the coverage range near saturation. Some of these structures can be tentatively described as (3 x 3(root 3)/2) rect and ((3 0)(12-5)). The surface coverages calculated from the different structures imaged by STM range from 0.44 to 0.50 Br/Pt, which are in agreement with those obtained by charge displacement and Auger data. This variation in coverage and structure indicates that near the saturation a compression of the Br adlayer on Pt exists, with a continuous variation of the interatomic distances in the adlayer, in the vicinity of the van der Waals diameter of bromine.