The drive here is to use high pressures and temperatures to make novel alloys, with direct band gaps and tailored lattice constants that act as effective interfaces between optical and electronic devices (for instance, link fibre optics to silicon chips). Ge-Sn binary alloys have been predicted to have such properties but unfortunately these elements are virtually immiscible at ambient pressure. Therefore this study is also of fundamental importance in developing new Ge-Sn phase relations, evaluating their formation properties, structures, stability and solution chemistry. The particular focus of this work is to explore mixing and reactivity of Ge and Sn at high pressures and high temperatures. Two pressure vessels were used, a piston-cylinder (up to 3.5 GPa and 1500 K) and a multianvil press (up to 24 GPa and 1500 K). The reaction products were processed (nano-processed when required with Focused Ion Beam methods). Mixing and reactivity of Ge-Sn was found to be dependent on the electronic properties of the starting elements at the experimental conditions. The two elements were found to separate below the Ge semiconductor-metal transition pressure (~9 GPa), but do not above this pressure.