Magnesium (Mg), and its alloys, is being investigated for its potential biomedical applications for its use as a biodegradable metal. However surface modification strategies are needed to modify the surface of the Mg alloy for its applicability in these applications. Self-assembled monolayers (SAMs) have been investigated as a coating strategy on magnesium for biomedical applications. In this report we evaluate the oxidative interfacial stability of phosphonic nanocoatings on magnesium using spectroscopic techniques. Self-assembled mono-/multilayers (SAMs) of octadecylphosphonic acid (ODPA) were formed on the native oxide layer of magnesium alloy using solution deposition technique. The SAMs modified Mg alloy and its oxidative stability were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). FTIR studies indicated mono-/bidentate bonding of the phosphonic SAMs to the Mg alloy surface. XPS confirmed SAM formation showing presence of "P" peaks while consequently showing decrease in peak intensity of Mg peaks. XPS analysis of the phosphonate peaks showed consistent presence of this peak over a period of 21 days. AFM images showed consistent coverage of the Mg alloy over a period of 21 days. The results collectively confirm that the monolayers are stable under the chosen oxidative study.