This proposal seeks to explore the polymorphism and polymerisation of methacrylic acid using high pressure. We have recently studied acrylic acid at high pressure using single crystal X-ray diffraction and neutron diffraction and and found that on application of pressure the low temperature form converts to a high pressure form which is stable to 7.0 GPa. The proposed experiment follows the compression of methacrylic acid. We have made preliminary studies of this system using single crystal diffraction and found it too has a high pressure phase. Initial experiments using neutron diffraction, however, have shown that a third transient phase can be formed although we have failed to identify the structure. This experiment will seek to characterise this phase and its stability with respect to pressure and the other two phases.