The photodimerization reaction of anthracene in supercritical CO(2) was studied systematically at different CO(2) densities. Unlike in normal liquid solvents, the reaction in supercritical CO(2) is significant even at anthracene concentrations as low as a few micromolar. At comparable anthracene concentrations, the photodimerization reaction is 1 order of magnitude more efficient in CO(2) than in normal liquid solvents. The dramatic increases in the photodimerization reaction efficiency in supercritical CO(2) may be explained by a mechanism in which the reaction is diffusion-controlled even when the diffusion rate constants are on the order of 10(11) M(-1) s(-1). The results also show that the efficient photodimerization reaction of anthracene is hardly affected by the local density augmentation (or solute-solvent clustering) in supercritical CO(2).