The rate of the Diels-Alder reaction between N-ethylmaleimide and 9-hydroxymethylanthracene in supercritical carbon dioxide (scCO(2)) was determined by following the disappearance of 9-hydroxymethylanthracene with in situ/vis absorption spectroscopy. The reaction conditions were 45-75 degreesC and 90-190 bar, which correspond to fluid densities (based on pure carbon dioxide) ranging between approximately 340 and 730 kg M-3. The measured reaction rate at low scCO(2) fluid densities was nearly 25x faster than that reported in acetonitrile at the same temperature (45 degreesC). An inverse relationship between reaction rate and fluid density/pressure was observed at all temperatures in scCO(2). The apparent activation volumes were large and positive (350 cm(3) mol(-1)) and only a weak function of reduced temperature. A solvophobic mechanism analogous to those observed in conventional solvents is postulated to describe (a) the rate acceleration observed for this reaction in scCO(2) relative to that in acetonitrile, (b) the observed relationship between reaction rate and pressure/temperature/density, and (c) the large, positive activation volumes. Solubility measurements in scCO(2), rate measurements in conventional solvents, and an empirical correlation are used to support this theory. Our results advance the general understanding of reactivity in supercritical fluids and provide a rationale for selecting reactions which can be accelerated when conducted in scCO(2).