The critical voltage for passivity breakdown on metals and steels in contact with halide-containing solutions is often observed to depend on anion size. In the case of iron and iron-base alloys, and for other metals and alloys (e.g., nickel-base and aluminum alloys), the greatest susceptibility toward passivity breakdown occurs with chloride ion. In this paper, an explanation of anion size effects is offered based upon the Point Defect Model. Briefly, the size effect is attributed to the competing effects of the energy and entropy (Gibbs energy) changes for expansion of a surface oxygen vacancy to a size that can accommodate the halide and the Gibbs energy of halide and possibly vacancy hydration/dehydration. The theory successfully accounts for the effect of anion size on the critical breakdown voltage of Fe-13Cr stainless steel. (C) 2016 The Electrochemical Society.