Supercritical fluids (SCFs), compounds heated and pressurized beyond the critical point, have many unusual properties. Homogeneous molecular catalysts, which have far greater control over selectivity than heterogeneous solid catalysts, are now being tested in SCFs, and early results show that high rates, improved selectivity, and elimination of mass-transfer problems can be achieved. As industry moves away from toxic or environmentally damaging solvents, supercritical carbon dioxide may be an ideal replacement medium for nonpolar or weakly polar chemical processes. More than simply substitutes for nonpolar solvents, SCFs can radically change the observed chemistry. Supercritical carbon dioxide is also an excellent medium for its own fixation, as demonstrated by studies of its hydrogenation.