Coated layers which formed on 7075-T6 aluminium alloy surfaces after treatment in zinc phosphate (ZPO) suspensions adjusted to different pH values (3.5, 5.0, 6.6, 10.5 and 13.0) were characterized by X-ray photoelectron spectroscopy (XPS). Comparative XPS tests were also made after the treated surfaces were exposed to a 3.5% NaCl solution for 5 h. The processes of coating from solution, oxidation, etching and bulk diffusion were all involved, although in different proportions, in the different treatments. The amounts of zinc and phosphorus in the uppermost layers were enhanced when the coating was at pH values of 5.0, 10.5 and 13.0, and in each case a mixed ZnO(x)-AlO(x)-ZPO structure was apparently formed. The coating prepared at pH = 13.0 appeared to provide the best corrosion protection.