The excitation of 3-hydroxyflavone (3HF) to its second excited singlet state (S-2) gives rise to dual fluorescence in supercritical carbon dioxide. The ultraviolet fluorescence originated from the S-2 state of 3HF is well separated from the green emission emanating from the tautomeric form, produced via the excited state intramolecular proton transfer. The relative intensity of the S-2 to the tautomer fluorescence (S-2/T) has been studied Lis a function of pressure and temperature. It is shown that this ratio reflects the microheterogeneity of the supercritical CO2, and confirms the value of fluorometric probes in disclosing the microscopic properties of supercritical fluids. (C) 2004 Elsevier B.V. All rights reserved.