The Diels-Alder dimerization of isoprene is an important reaction; dipentene (or racemic limonene), one of the products formed in this reaction, has major applications in the manufacture of polymers and adhesives.’ Dipentene also has various uses in the food and pharmaceutical industries.(7) In the present work, the QCFF force field program(2) was used to calculate gas-phase thermodynamic properties of the monomer (isoprene) and the dimers 1-methyl-5-(1-methylethenyl)cyclohexene (diprene) and 1-methyl-4-(1-methylethenyl)cyclohexene (dipentene) for the temperature range 298.15-1000 K. These QCFF-calculated thermodynamic values were compared, when possible, with corresponding values obtained experimentally or from other force field programs, and the agreement was found to be satisfactory. The QCFF values were further used to derive gas-phase equilibrium properties-Delta H degrees, Delta S degrees, Delta G degrees, and K-p-for the isoprene dimerization reaction at various temperatures. These computational results suggest that, based upon thermodynamics, diprene and dipentene are about equally favored dimerization products of isoprene and that reported experimental data reflect kinetic control of dipentene formation at lower temperatures. The equilibrium thermodynamics data for the isoprene dimerization reaction presented in this paper are felt to be sufficiently reliable that they can be utilized in the absence of any additional experimental data; the present work thus illustrates the power of predictive thermodynamic computational techniques in extending the thermochemical database.