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 0.2mA/cm2 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 0.05mA/cm2. The ionic liquid is shown to support oxygen reduction currents of greater than 0.8mA/cm2, suggesting that this electrolyte is a suitable candidate for magnesium-air battery applications.