Ab initio and density functional calculations using a variety of theoretical methods (CASSCF, B3LYP, CASPT2, CCSD(T), and G3(MP2,CC)) have been carried out to unravel the mechanism of unimolecular isomerization and dissociation of 9,10-dihydrofulvalene C(10)H(10) (S0) formed by barrierless recombination of two cyclopentadienyl radicals. Different reaction pathways on the C(10)H(10) potential energy surface (PES) are found to lead to the production of 9-H-fulvalenyl radical + H, 9-H-naphthyl radical (a naphthalene precursor) + H, and naphthalene + H(2). RRKM calculations of thermal rate constants and product branching ratios at the high pressure limit show that at temperatures relevant to combustion the 9-H-fulvalenyl radical formed by a direct H loss from S0 with endothermicity of 76.3 kcal/mol is expected to be the dominant reaction product. The naphthalene precursor 9,10-dihydronaphthalene (D3) can be produced from the initial S0 adduct by a multistep diradical mechanism involving the formation of a metastable tricyclic diradical intermediate, followed by its three-step opening to a 10-member ring structure, which then undergoes ring contraction producing the naphthalene core structure in D3, with the highest barrier on this pathway being 70.3 kcal/mol. D3 can lose molecular hydrogen producing naphthalene via a barrier of 77.7 kcal/mol relative to the initial adduct. Another possibility is a hydrogen atom elimination in D3 giving rise to the 9-H-naphthyl radical without exit barrier and with overall endothermicity of 59.2 kcal/mol. The pathway to 9-H-naphthyl appears to be preferable as compared to the direct route to 9-H-fulvalenyl at temperatures below 600 K, but the rate constants at these temperatures are too slow for the reaction to be significant. The naphthalene + H(2) channel is not viable at any temperature. The following reaction sequence is suggested for kinetic models to account for the recombination of two cyclopentadienyl radicals: c-C5H5+c-C5H5-->9,10-dihydrofulvalene-->9-H-fulvalenyl+H(C10H10PES), 9-H-fulvalenyl-->naphthalene+H/fulvalene+H(C10H9PES). We conclude that naphthalene can be produced from the recombination of two cyclopentadienyl radicals and is expected to be a favorable product of this reaction sequence at T < 1000 K, but this molecule would be formed through isomerizations and H atom loss on the C(10)H(9) PES (after the initial H elimination from C(10)H(10) S0) and not in conjunction with molecular hydrogen. The alternative product, fulvalene, can potentially contribute to the growth of cyclopentafused polycyclic aromatic hydrocarbons.