Heterogeneous strong base catalysis for the intramolecular Tishchenko reaction of aromatic 1,2-dicarbaldehydes to the corresponding phthalides in supercritical CO(2)CscCO(2) has been realized with mesoporous alumina containing SO42- ions in the alumina framework (mesoAl(2)O(3)/SO42-). Infrared spectroscopy of pyrrole adsorbed on the alumina and strong poisoning by a weak Bronsted acid of methanol revealed that the SO42- ions in the framework slightly suppressed the average strength of base sites (O2-) on mesoAl(2)O(3)/SO42-, but there exists a small number of strong base sites that promote the Tishchenko reaction in SCC02. Although the intramolecular Tishchenko reaction of phthalaldehyde to phthalide in SCC02 was somewhat slower than those in organic solvents such as tetrahydrofuran (THF) and benzene, the addition of a small amount of THF as a cosolvent remarkably increased the reaction rate; the reaction in the scCO(2)-THF system proceeded 1.5-fold faster than those in pure benzene and THF solvents.