The photosensitized production and subsequent decay of singlet oxygen in supercritical fluid carbon dioxide has been studied as a function of pressure and temperature. The rate of decay is shown to be a function of the fluid density, demonstrating a quenching effect by the solvent. The observed rate constant for decay is also found to depend on the oxygen concentration, as a result of quenching of singlet oxygen, O-2(a(1)Delta (g)) by ground state oxygen. The rate constant for quenching by oxygen is found to be 1.9 x 10(4) dm(3) mol(-1) s(-1), at 41 degreesC and at a pressure of 150 kg cm(-2), which is compared with previous findings in conventional solvents. The lifetime of singlet oxygen in supercritical fluid carbon dioxide, under these conditions, extrapolated to the value in the absence of ground state molecular oxygen, was found to be 5.1 ms (at 150 kg/cm(2) and 41 degreesC). The factors determining the lifetime of O-2(a(1)Delta (g)), in supercritical fluid carbon dioxide are discussed.