The adsorption isotherms for chloride on a corroding aluminium surface have been measured by using 36Cl as a radioactive tracer. The amount of chloride adsorbed, wCl, was a function of chloride concentration, [Cl], and time, t, according to ������������ �����������log WCl = 0.64(log[Cl]+log t)-7.8 where wCl is expressed as g cm-2, [Cl] as mol l-1 and t as minutes. Superimposed on the general curve was a fine structure which correlated with the stages of development of a corroding aluminium surface. The adsorption was localized to corroding pit sites. Addition of nitrate or sulphate delayed but did not prevent the uptake of chloride; the hydroxide ion was considered to have a similar effect. It was concluded that a corroding aluminium surface has a variety of adsorption sites with different adsorption properties; only a minority of these sites are potential or active sites for pitting corrosion.