Magnesium-air batteries are a promising high-energy density power source that utilises an active magnesium electrode and oxygen. Herein, the magnesium surface formed during 0.2 mA/cm(2) galvanostatic discharge in trihexy(tetradecyl)phosphonium chloride ionic liquid electrolyte was characterised using FTIR, XPS and mass spectroscopy and the complex nature of the interfacial film formed is described. This film has a conductive gel-like structure and consists of hydrated organophosphonium magnesium(hydroxyl)chlorides. A laboratory synthesised form of the gel showed comparable discharge behaviour and was able to support a discharge current of 0.05 mA/cm(2). The ionic liquid is shown to support oxygen reduction currents of greater than 0.8 mA/cm(2), suggesting that this electrolyte is a suitable candidate for magnesium-air battery applications. (C) 2012 Elsevier Ltd. All rights reserved.