We propose to use GEM to study the evolution of both the crystal and magnetic structures of Ba0.85K0.15O2 between room temperature and 2 K. Ba0.85K0.15O2 is a novel phase not previously reported in the literature and contains unpaired p-electrons localised on dioxygen anions; bulk magnetization measurements indicate weak ferromagnetism to well above room temperature, unprecedented for a p-electron magnetic system. We plan to use GEM to probe the nature of this anionogenic spin ordering. We will also obtain detailed information on the crystal structure as a function of temperature, focusing in particular on the spatial arrangement and orientation of the dioxygen anions, which are currently unknown. This will help us to account for the remarkable magnetic properties.