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Abstract: Solubilities of the xanthines caffeine, theophylline, and theobromine in different supercritical (carbon dioxide + methanol) mixtures were measured at 313 K and 22 MPa. The solubilities of theobromine in supercritical carbon dioxide + methanol were also measured at 333 K and 22 MPa. The determination of solubilities has been performedbyusing a static analytical methodwith directcouplingof an equilibrium cell to a supercritical fluid chromatographic system. The solubility of all xanthines was enhanced by adding methanol to the fluid. For example, with 0.1 mass fraction of methanol in the fluid mixture the solubility of every xanthine increases by 1 order of magnitude. Although the chemical structures of the xanthines are very similar, their solubilities in supercritical carbon dioxide mixtures, as in pure carbon dioxide, vary substantially. The solubilities of caffeine in CO2 are 1 order of magnitude higher than those of theophylline and 2 orders of magnitude higher than those of theobromine.