As an intermediate form between homogeneous and heterogeneous catalysis, catalysts based on transition metal colloids have drawn a lot of attention in recent years. Mating advantages from different concepts of catalysis, they turn out to be highly active, selective, and, in addition, easily separated from other reaction components. Even though colloidal catalysts are widely used in conventional solvents, hardly anything is known about their reactivity in supercritical fluids such as supercritical carbon dioxide (ScCO2). Furthermore, scCO(2) is an especially attractive, non-toxic, and environmentally benign solvent for chemical reactions. When hydrogenations were conducted using polymer-supported colloidal Pd nanoparticles as catalysts in scCO(2), we found turnover frequencies (TOFs) as high as 4 000 000 h(-1) even at a reasonably low hydrogen pressures of 15 bar and temperatures of 50 degreesC. To our surprise, their reactivity turned out to be much higher than those of most other catalysts reported in the literature. The kinetics of their catalytic reactions in supercritical fluids has been investigated using in situ NMR in combination with a toroid cavity autoclave (TCA). (C) 2002 Elsevier Science B.V. All rights reserved.