An overlayer of chemisorbed arsenic, As-chem, On Pt(111) having a surface coverage of theta (As) is prepared, and the impact of T variation on the surface-oxidation behavior in 0.5 M aqueous H2SO4 is examined. Temperature variation in the 273-313 Krange does not affect the AS(chem) surface coverage, but the cyclic-voltametry (CV) profiles undergo qualitative changes. The anodic and cathodic peaks shift toward more-positive potentials, and a charge-density redistribution between two anodic and cathodic features is observed, the latter effect being more pronounced in the anodic scans. At T = 313 K, the CV transients are symmetrical with respect to the potential axis. In the case of AS(chem) overlayers having theta (As) well below 0.33, the CV profiles representative of the AS(chem) oxidation reveal only one peak. An analysis of the possible reaction pathways indicates that the surface oxidation proceeds in one step involving transfer of three electrons and addition of three OH groups, thus in formation of As(OH)(3chem). An analysis of the As3+ and OH- radii indicates that the As(OH)(3chem) layer is densely packed. The existence of two anodic and two cathodic peaks in CV transients is explained in terms of formation two energetically slightly different structures of AS(OH)(3chem), their origin being two different ways of coordinating AS(OH)(3chem) to Pt(lll). The standard enthalpy of the surface process is determined from the slope of the E/T versus 1/T plots, and the standard enthalpy of formation of As(OH)(3chem), DeltaH(f)(o)(As(OH)(3chem)), is found to be -678 kJ mol(-1). The value of DeltaH(f)(o)(As(OH)(3chem)) is characteristic of As being in the 3+ oxidation state and being attached to three OH groups. The conclusion that AS(OH)(3chem) is the species formed agrees with the CV, IR spectroscopy, and ex-situ scanning tunneling microscopy data which point to an oxygenated As3+ surface compound formed in the course of AS(chem) electro-oxidation.