The isoelectric point of the oxide-covered aluminum surface is an important parameter in the pitting behavior of aluminum and of aluminum binary surface alloys. Contact angle measurements show that ion implantation of Ta into Al modifies the isoelectric point of the resulting oxide film to a value which is intermediate between that of Al2O3 and Ta2O5. The critical pitting potential is shown to be a function of the flatband potential of the oxide-covered aluminum surface. For a series of alloys where the matrix metal (and host oxide) is held fixed, the flatband potential is a function of the isoelectric point of the oxide film on the alloy surface, which in turn is a function of the isoelectric point of the oxide of the alloying element. Thus, the critical pitting potential of a binary surface alloy is related to the isoelectric point of the oxide of the alloying element in the binary alloy. Surface alloying affects the pitting potential by (i) modifying the isoelectric point of the surface so as to change the surface charge, (ii) affecting the heat of adsorption of the chloride ion, and (iii) changing the concentration of oxygen vacancies in the oxide film.