The kinetics of the reaction of maleic anhydride and isoprene in supercritical carbon dioxide was studied over an extended concentration range, from 5 x 10(-5) to 1.3 x 10(-2) mote fractions of both maleic anhydride and isoprene at 60 degreesC and pressure 100 atm. The apparent rate constant of the reaction was found to be concentration dependent, in contradiction with the literature data. The results are interpreted in terms of limited solubility of maleic anhydride in supercritical CO2. The solubility of maleic anhydride in supercritical carbon dioxide was directly measured by sampling and analyzing the supercritical phase. The observed sharp increase in the apparent rate constant of the reaction between maleic anhydride and isoprene correlates with the solubility of maleic anhydride in supercritical carbon dioxide. The interpretation was confirmed using visual observations of the phase behavior. The rate constant of the reaction measured under the true supercritical, single phase, conditions is k(x) = (17.0 +/- 1.3) hr(-1) (mole fraction units, 60 degreesC, pressure 100 atm).