The cyclopentadienyl radical (cC(5)H(5)) is among the most stable radical species that can be generated during the combustion and pyrolysis of hydrocarbons and it is generally agreed that its contribution to the gas phase reactivity is significant. In this study the kinetics of one key cC(5)H(5) reaction channel, namely the reaction between cC(5)H(5) and cyclopentadiene (cC(5)H(6)), was investigated using ab initio calculations and RRKM/Master Equation theory. It was found that most of the excited C5H5_C5H6 adducts formed by the addition of cC(5)H(5) to cC(5)H(6) decompose back to reactants and that the major reaction products are, in order of importance, indene, vinylfillvene (a most probable styrene precursor), phenylbutadiene, and benzene. The preferred reaction pathway of the C5H5_C5H6 adduct is started by the migration of the tertiary hydrogen of the C5H5 ring to a vicinal carbon and followed by the beta-opening of the C5H6 ring, which is the rate determining step. Successive molecular rearrangements lead to decomposition to the four possible products. The kinetic constants for the four reaction channels, calculated at atmospheric pressure and interpolated in cm(3) mol(-1) s(-1) between 900 and 2000 K, are k(indene) = 10(25.197)T(-3.935) exp(-11630/T(K)), k(vinylfulvene) = 10(65.077)T(-14.20) exp(-37567/T(K)), k(benzene) = 10(29.172)T(-4.515) exp(-20570/T(K)), and k(phenylbutadiene) = 10(16.743)T(-1.407) exp(-11804/T(K)). The predictive capability of the reaction set so determined was tested through the simulations of recent cC(5)H(6) pyrolysis and combustion experiments using a detailed kinetic mechanism. A quantitative agreement with experimental data was obtained by assuming that vinylfulvene converts rapidly to stryrene, increasing its reaction channel by a factor of 2, and assuming that phenylbutadiene rapidly decomposes with equal probability to styrene and benzene.