Understanding the processes leading to phase-, chemical- and isotopic separation in high temperature and high pressure geochemistry are increasingly turning to 'non-traditional' methods. These include, but are not limited to, the fractionation of elements and their stable isotopes (the ‘non-traditional’ stable isotopes, those heavier than O) via diffusive, evaporative, convective, or mechanical mass transfer processes between major planetary reservoirs (core, mantle and crust) and planet-forming materials. As measurements of non-traditional isotopes in natural samples continue to proliferate, their fractionation, which is measurable even at high temperatures, necessitates quantitative understanding of the mechanisms that engender this fractionation. In recent years, elemental and isotopic fractionation at high pressures and temperatures relevant to Earth’s interior has now become measurable using state-of-the-art experimental (e.g., diamond anvil cell) and numerical (e.g., ab-initio) methods. We invite contributions that attempt to shed light on mass transfer processes relevant to geochemistry at high temperatures and pressures. This involves isotope or element partitioning studies in natural rocks, high temperature-high pressure experiments, ab-initio calculations and numerical models.