When placed under ~10 GPa of pressure, Si and Ge are no longer semiconductors as they phase transform into a metallic structure, similar to white Sn (β-Sn). When the pressure is released, both materials don’t change back to the original diamond cubic structure but rather to a series of metastable phases (such as r8, bc8, st12 and amorphous). Each of these structures has different properties to the original diamond cubic structure. Whilst these metastable structures are known to exist, the pathways to formation are not well understood as many factors play a role in determining the final metastable phase; such as size, temperature, shear, the purity of the material, and the rate at with pressure is applied and released. Here, the impact of two parameters, temperature and size, on the formation of the resulting metastable phases of Si and Ge is examined. The effect of temperature is examined using an in-situ temperature stage during nanoindentation. Due to the nature of the pressure application in nanoindentation the nucleation of defects is a mechanism which competes with phase transformation. At room temperature, defect propagation is dominant. Thus, the lowering of temperature as a method to overcome defect propagation is proposed and any metastable phases formed are identified. To investigate the influence of size, the high-pressure behaviour of a range of Si nanowires are studied using diamond-anvil cells. These results show that the smaller size of the nanowires cause them to become more difficult to phase transform than bulk-Si due to nucleation issues.