The thermal isomerizations of bicyclopentene (bcp) and tricyclopentane (tcp) into cyclopentadiene (cp) are investigated by a combination of DFT, CASSCF, CASSCF-MP2, and CR-CCSD(T) methods. Coupled-clusters and B3LYP methods predicted the reaction enthalpies excellently whereas the MCSCF method worked well only when dynamic correlation energy was taken into account. Both processes are concerted, and the reaction paths pass through transition states with high biradical character. Measures of biradical character in DFT and ab initio methods are discussed. The activation enthalpy in the rearrangement of bcp into cp was predicted by the CR-CCSD(T) method to be 25.5 kcal/mol, in good agreement with experiment. The UB3LYP functional also performed well in this case despite the high spin contamination that was present in the singlet biradicaloid transition state. The reaction enthalpy for the conversion of tcp into cp was predicted to be -63.7 kcal/mol. The transition state involved in the isomerization of tcp was found to be nearly degenerate with the triplet state causing less certainty in the estimated activation enthalpy of 48.3 kcal/mol.