Supercritical (sc) fluids have unique properties that have led to a rich variety of applications including production of essential oils and pharmaceutical products, green chemistry, formation of nano- and micro-particles, and waste treatment. The proposed work will examine the microscopic structure and molecular motion in supercritical (sc) CO2. The results from this work will provide the first experimental evidence for a structural crossover in a new dynamic line in the supercritical region of the phase diagram, the Frenkel line [1-2]. This work will be enhanced by the use of classical molecular dynamics to provide information on the rotational motion of scCO2.