THE use of carbon dioxide as a starting material for the synthesis of organic compounds has long been a goal for synthetic chemists. The hydogenation of carbon dioxide to formic acid, methanol and other organic substances is particularly attractive, but has remained difficult. This route to formic acid has been described recently, based on the use of organometallic rhodium catalysts in dimethyl sulphoxide(1) and aqueous(2) solvents. We report here the efficient production of formic acid in a supercritical mixture of carbon dioxide and hydrogen containing a catalytic ruthenium(II) phosphine complex. The use of a supercritical phase, in which hydrogen is highly miscible, leads to a very high initial rate of reaction-up to 1,400 moles of formic acid per mote of catalyst per hour. The same reaction under identical conditions but in liquid organic solvents is much slower. Our results suggest that supercritical fluids represent a promising medium for homogeneous catalysis.