Irreversibly adsorbed Co adlayers with surface coverages of about 0.6 and 1 ML can be deposited onto Pt(lll) from CoSO4 in 0.1 M H2SO4 and CoCl2 in 0.1 M NaOH solutions. Once adsorbed, the adlayers were studied in 0.1 M H2SO4 and 0.1 M NaOH solutions respectively, by electrochemical, surface EXAFS (SEXAFS) and crystal truncation rod (CTR) measurements. In 0.1 M H2SO4, an irreversible oxidation with a peak potential value of +1.00 V (vs. Ag/AgCl) corresponding to the oxidation of the cobalt adlayer as well as the Pt substrate was observed. There was also a reduction wave present at -0.29 V and which was ascribed to, at least in part, the evolution of hydrogen and which resulted in the displacement of the cobalt adlayer. In 0.1 M NaOH two redox processes were also observed at formal potential values of -0.355 and -1.04 V. The former, which exhibited significant kinetic limitations, was ascribed to a Co-III(OH)(3)/Co-II(OH)(2) redox process. The latter was ascribed to the generation of Co-0. SEXAFS measurements in sulfuric acid (at an applied potential of 0.0 V) and in 0.1 M NaOH (at applied potential values of -0.06 and -0.75 V) were consistent with the presence of an oxidized cobalt layer with Co, O and Pt near neighbors. CTR data were also consistent with the presence of an oxidized cobalt bi-layer structure. In 0.1 M NaOH and at a coverage of ca. 1 ML, the deposited cobalt appears to form a 1 x 1 structure with the cobalt atoms occupying three-fold hollow sites. Relative changes in the number of oxygen near neighbors with applied potential (derived from EXAFS data) were also consistent with electrochemical data. In 0.1 M H2SO4 (at an applied potential of 0.0 V) and at a cobalt coverage of about 0.6 ML, the deposited cobalt appears to form an incommensurate monolayer with an expanded lattice (Co-Co distance of 2.96 Angstrom) likely involving co-adsorbed (bi)sulfate.