Thermal decomposition of cyclopentadiene to c-C5H5 (cyclopentadienyl radical) + H (1) and the reverse bimolecular reaction (-1) are studied quantum-chemically at the G2M level of theory. The dissociation pathway has been mapped out following the minimum energy path on the potential energy surface (PES) calculated by the density functional UB3LYP/6-311G(d,p) method. Using isodesmic reaction analysis, the standard enthalpy of formation for c-C5H5 is found to be 62.5 ± 1.3 kcal mol-1, and the c-C5H5-H bond dissociation energy is estimated as D°298(c-C5H5-H) = 82.5 ± 0.9 kcal mol-1, in excellent agreement with the recent experimental values. Variational rate constants are computed on the basis of a scaled UB3LYP dissociation potential that fits the isodesmic/experimental enthalpy of Reaction (1). At the high pressure limit, k1 ∞ = 1.55 × 1018 T-0.8 exp(-42300/T) s-1 and k-1 ∞ = 2.67 × 1014 exp(-245/T) cm3 mol-1 s-1. The fall-off effects are evaluated by a weak collision master equation/RRKM analysis. Calculated T, P-dependent rate constants are in very good agreement with the most reliable experimental measurements.