Oxygen is remarkable in that it has a triplet ground state, a 3Σg state, with its first two excited states being of singlet multiplicity (1Δg and 1Σg+, lying 94 kJ mol-1 and 158 kJ mol-1 above the ground state, respectively). Relaxation of the singlet state to the ground state is consequently spin forbidden, resulting in a long lifetime for singlet oxygen: Singlet oxygen is a highly reactive species and is responsible for oxidative damage in a number of systems. In this study, singlet oxygen quantum yields, ΦΔ, have been measured at varying temperatures and pressures, as a function of oxygen concentration, in supercritical fluid carbon dioxide.