Tricyclopentadiene (TCPD), product of dicyclopentadiene (DCPD)/cyclopentadiene (CPD) cycloaddition, is a promising candidate as high-energy-density fuel. Thermal [4 + 2] cycloaddition of DCPD and CPD was simulated using M05-2X/6-311G(d,p) to predict the product distribution. There are four concerted but slightly asynchronous pathways leading to four adducts for endo-DCPD/CPD and exo-DCPD/CPD additions, respectively. The pathways connecting endo-adduct are kinetically preferred compared with those connecting exo-adduct. NB-adducts show higher stability than corresponding CP-adducts. Moreover, exo-isomers are more stable for CP-adducts, but the tendency is inverse for NB-adducts. The predicted product preference is NB-endo > CP-endo > CP-exo > NB-exo. Experiment shows product composition of NB-endo: CP-endo = 7.32: 1 and NB-endo: CP-endo: CP-exo = 3.79: 1: 0.1 for thermal endo-DCPD/CPD and exo-DCPD/CPD additions, respectively, consistent with the predicted result. However, catalytic reaction gives completely different distribution, in which [4 + 2] NB-exo and CP-exo, along with at least 21.6% [2 + 2] adducts, are formed. (C) 2009 Elsevier Ltd. All rights reserved.