Phenyl-C61-butyric acid methyl ester (PCBM) is a fullerene-based material of high technological interest due to its use in organic solar photovoltaic cells. A better understanding of its thermal properties is needed to provide fresh insights into the prospects of higher energy-conversion efficiencies for this family of organic materials. Following a successful Xpress run on IRIS, this proposal seeks to provide an unambiguous identification and detailed characterisation of a plastic/rotor phase in PCBM above 360K, analogous to that which is found in C60. In conjunction with ongoing molecular-dynamics simulations, these experimental data will also be used to understand PCBM's extremely low (and hitherto unexplained) thermal conductivity.