The aim of this project is the single crystal structure determination and characterisation of phase transitions of crystalline carbonic acid (H2CO3) at high pressures. Understanding structure-property relations of H2CO3 is of significant interest not only to astrophysics, as H2CO3 is likely to be present on the surface and in the interiors of icy moons (e.g. Ganymede) or ice giants (e.g. Uranus), but also to solid state chemistry, as it is a molecular compound which has led to a number of mutually contradictory studies, e.g. regarding an assumed polymorphism. Up to now no single crystal diffraction data of H2CO3 has been reported and its structure even at low pressures (< 10 GPa) is in dispute. In preliminary experiments, we synthesized H2CO3 crystals from a H2O + CO2 mixture and characterised the product by Raman spectroscopy. We now propose to determine the structure of H2CO3 in a wide pressure range (1-150 GPa).