The localised corrosion of AZ31 magnesium alloy immersed in chloride-containing electrolyte is characterised by the time-dependent evolution of dark, dendrite-like features comprising several mobile, focal anodes which are coupled with strong local cathodes situated at corroded regions left behind advancing anodes. The uncorroded, film-covered AZ31 surface is shown to be electrochemically inert in comparison. The number and intensity of the local anodes increase with time in proportion to the area of exposed local cathode, indicating that localised corrosion propagates under cathodic control. When the entirety of the exposed surface becomes affected by a network of dark dendritic corrosion features, the localised corrosion rate becomes time-independent although local anodes remain mobile local and traverse a cathodically activated corroded alloy surface. The source of this cathodic activation is proposed to be an enrichment in noble Al-Mn particles as anodic dissolution of the alpha Mg phase occurs. Localised corrosion rates are largely insensitive to pH in the 4 <= pH <= 10 range, but appreciably higher at pH 2 and markedly lower at pH values of 12 and above. The time taken for localised corrosion to initiate is independent of pH, an observation which appears inconsistent with the theories of elevated bulk pH acting to reinforce a protective (hydr)oxide film. Progressively lower post-breakdown local cathodic current density values are observed with increasing alkalinity, as a consequence of the pH dependence of the hydrogen evolution equilibrium potential. (C) 2013 Elsevier Ltd. All rights reserved.