The paper studies some aspects of the behaviour of four aluminium alloys under chemical etching by sodium hydroxide solution and during their subsequent anodizing in sulphuric acid solution. A correspondence is seen between etching rate, thickness of the passivating oxide film and porosity of the anodic layer. The possibility of an influence on these properties of precipitates and micro-heterogeneities in the metallic surface is suggested. (c) 2007 Elsevier B.V. All rights reserved.