AlF₃ and MgF₂ are important catalysts for fluorination processes. In this study we characterise structures consisting of either AlF₃, or MgF₂ supported over AlF₃, using Fourier transform infrared spectroscopy of surface absorbed CO. We interpret the results using atomic-scale density functional calculations, along with high-resolution electron microscopy and spectroscopy. By coupling theory to vibrational spectroscopy we are able to establish different stable facets in the host species, identifying both surface physisorbed CO binding to pristine facets, and chemisorbed CO binding to fluorine anion vacancy sites. In the case of MgF₂ supported over AlF₃ surfaces we suggest that a 50 : 50 mixed Mg-AlF_x monolayer surface phase can form. Excess MgF₂ deposition subsequently results in the formation of MgF₂ nanoparticles on the surface. Mixed composition surface phases may play a critical role in Al/MgFx fluorine catalysis.