The mechanism of the a.c. electrolytic graining of aluminium in hydrochloric acid is determined from the analysis of the potentiostatic transient behaviour of the system aluminium-electrolyte under anodic and cathodic polarization and comparison of experimentally determined transients with calculated values derived from a candidate mechanistic scheme. It has been established, that the oxidation of aluminium in the development of a distinct surface morphology occurs according toAl + 3 Cl- (k(1)) under right arrow Al(Cl)(3) + 3e(-)Al(Cl)(3) (k(2)) under right arrow Al3+ + 3 Cl-the Al3+ ions being dissolved from the surface and removed to the bulk of the solution, hence forming pits. Al(Cl)(3) is a solid intermediate. The morphology developed, is determined by the excess of Cl- ions created at the electrode surface, with respect to the bulk concentration. The accumulation of Cl- ions is governed by the ratio between the rate constant for the formation of Al(Cl)(3), set by the flux of charges forced across the electrode-solution interface per unit surface area taking part in the active dissolution of aluminium and the mass transport rate of the Cl- ions. The reduction of H+ ions in the cathodic half period of the applied alternating current is mass transport controlled. The concomitant rise in interfacial pH causes Al3+ ions formed in the preceding anodic half period, which are not yet removed from the electrode-solution interface, to precipitate as aluminium.