This work investigates the diffusion of metal atoms into phase change chalcogenides, which is problematic because it can destroy resonances in photonic devices. Interfaces between Ge2Sb2Te5 and metal layers were studied using X-ray reflectivity and reflectometry of metal-Ge2Sb2Te5 layered stacks. The diffusion of metal atoms influences the crystallisation temperature and optical properties of phase change materials. When Au, Ag, Al, W structures are directly deposited on Ge2Sb2Te5, inter-diffusion occurs. Indeed, Au reacts with Ge2Sb2Te5 to form a AuTe2 layer at the interface. Diffusion barrier layers, such as Si3N4 or stable plasmonic materials, such as TiN, can prevent the interfacial damage. This work shows that the interfacial diffusion must be considered when designing phase change material-tuned photonic devices, and that TiN is the most suitable plasmonic material to interface directly with Ge2Sb2Te5.