The effect of pressure on the measured bimolecular rate constant of the Diels-Alder reaction between maleic anhydride and isoprene was investigated in supercritical CO2 and sub-critical propane. The reaction was carried out at 35 degrees C in CO2 and 80 degrees C in propane. Measured bimolecular rate constants are also compared to predictions from the thermodynamic pressure effect using transition state theory and the Peng-Robinson equation of state. The rate constants in supercritical CO2 agreed closely with the thermodynamic pressure effect predictions over the entire pressure range. Furthermore, the mole fraction based rate constants were found to vary linearly with the density of the solution. The rate constants in the sub-critical propane solvent significantly diverged from the thermodynamic pressure effect predictions and were found to deviate from this linear density dependence at the lower pressures studied. The results are interpreted in the context of local reactant concentrations about the reacting maleic anhydride and solvent-solute and cosolvent-solute interactions.