The surface reconstruction of Au(100) and the formation of an ordered commensurate c(root2 x 22root2)R45degrees adlayer of Br (Br-ad) have been studied by a combination of electrochemical (EC) and surface X-ray scattering (SXS) measurements. Emphasis is placed on linking the microscopic structural information concerning the Br-ad adlayer to the voltammetric and other macroscopic electrochemical responses, including using the rotating ring disk electrode (RRDE) measurements for determining the surface coverage by Br-ad. It is found that the potential-induced hexagonal ("hex") to (1 x 1) transition of the An surface coincides with Br- adsorption and occurs faster in solutions containing Br- anions than in Br- free Solutions. In agreement with previous SXS results, in acid solutions, Br-ad forms a c(root2 x 2root2)R45degrees structure at around 0.15 V. However, no ordered structures of Br-ad are observed in alkaline solution, although the cyclic voltammetry indicates that the order structure should be present at 0.12 V. Absence of an ordered Br-ad adlayer in alkaline solution is attributed to competitive adsorption between Br-ad and OHad. To probe the role of OHad on the ordering of the bromide adlayer, coadsorbed OHad is consumed in an electrochemical reaction in which strongly adsorbed OHad is removed from the surface by a relatively weakly adsorbed reactant, viz. COad. Under such experimental conditions, we found that (i) in acid solution the c(root2 x 2root2)R45degrees structure develops/disappears more rapidly than in CO-free solution and (ii) in alkaline solution the c(root2 x 2root/2)R45degrees structure is formed in exactly the same potential region as in acid solution. We propose that the continuous removal of OHad in the Lanmmuir-Hinshelwood reaction (COad + OHad = CO2 + H+ + e(-)) may stabilize the c(root2 x 2root2)R45degrees structure in both acid as well as alkaline media.