Different from from traditional methods for preparing pH-switchable Pickering emulsifiers, a simple and straightforward approach is established on the basis of a reversible process between in situ formation and dissolution of Mg(OH)(2) nanoparticles (MHp). It was found that when pH value was above 9.5, emulsions of liquid paraffin-in-water can be stabilized by the resulting surface-active particles. Below this pH, emulsions demulsify, resulting in a reversible Pickering emulsifier. Based on the strongly pH-dependent precipitation of metal hydroxide nanoparticles, this procedure offers a new way to design pH-switchable emulsifiers without aid of any other organic matters. (C) 2010 Published by Elsevier Inc.