The effects of pH on the surface reconstruction of Au(100), on CO oxidation, and on the oxygen reduction reaction (ORR) have been studied by a combination of surface X-ray scattering (SXS), Fourier transform infrared (FTIR) spectroscopy, and rotating ring-disk electrode (RRDE) measurements. In harmony with previous SXS and scanning tunneling microscopy (STM) results, the potential-induced hexagonal ("hex") to (I x 1) transition occurs faster in an alkaline electrolyte than in acidic media. In alkaline solution, CO adsorption facilitates the formation of a "hex" phase; in acid solution, however, CO has negligible effect on the potential range of thermodynamic stability of the "hex" <----> (1 x 1) transition. We propose that in KOH the continuous removal of OHad in the Langmuir-Hinshelwood reaction (CO + OH = CO2 + H+ + e(-)) may stabilize the "hex" phase over a much wider potential range than in CO-free solution. In acid solution, where specifically adsorbing anions cannot be displaced by CO from the Au(100) surface, CO has negligible effect on the equilibrium potential for the "hex" <----> (1 x 1) transition. Such a mechanism is in agreement with the pH-dependent oxidation of CO. The ORR is also affected by the pH of solution. It is proposed that the pH-dependent kinetics of the ORR on Au(100) can be unraveled by finding the relationship between kinetic rates and two terms: (i) the energetic term of the Au(100)-O-2(-) interaction determines the potential regions where the rate-determining step O-2 + e = O-2(-) occurs, and (ii) the preexponential term determines the availability of active sites for the adsorption of O-2(-).