Abstract: A method of treating aluminium, particularly as a means of anodising an aluminium lithographic plate, which method forms an anodised layer which has improved porosity, thickness, abrasion resistance, and chemical inertness but physical adhesion to overcoatings; the method comprises the steps of firstly anodising the aluminium by electrolysis in sulphuric acid solution, and secondly, anodising the aluminium by electrolysis in phosphoric acid solution. The same anodising potential, preferably of 10 to 15 volts d.c. can be used for both steps, which are substantially continuous with one another. The surface is preferably grained first, by electrolysis in dilute hydrochloric acid. Preferred values of specific gravity for the sulphuric and phosphoric acids are given.