The enthalpies of formation were determined for 15 intermediate species of cyclopentadiene oxidation under combustion conditions by ab initio molecular orbital calculations at the G2(MP2,SVP) and G2(B3LYP/MP2,SVP) levels of theory and with the use of isodesmic reactions, The G2(B3LYP/MP2,SVP) method, a hybrid of GZ(MP2,SVP) and the density-function-based G2(B3LYP/MP2), was devised in the present study. The devised method is less demanding computationally, but it provides atomization energies with accuracy comparable with G2(MP2) and G2(B3LYP/MP2). In addition. the pathways and reaction rate coefficients for the thermal decomposition of cyclopentadienone were studied with molecular orbital and RRKM calculations. The computational results show that at high temperatures the decomposition of cyclopentadienone leads primarily to cyclobutadiene. An analysis of the chemically activated reactions of the cyclopentadienyl radical suggests that cyclopentadienylidene may potentially be an important intermediate species of cyclopentadiene oxidation at high temperatures.