The thermal decomposition of cyclopentadienone (C5H4=O) has been studied in a flash pyrolysis continuous flow microreactor. Passing dilute samples of o-phenylene sulfite (C6H4O2SO) in He through the microreactor at elevated temperatures yields a relatively dean source of C5H4=O. The pyrolysis of C5H4=O was investigated over the temperature range 1000-2000 K. Below 1600 K, we have identified two decomposition channels: (1) C5H4=0 (+ M) --> CO + HC=C-CH=CH2 and (2) C5H4=O (+ M) --> CO + HC=CH + HC=CH. There is no evidence of radical or H atom chain reactions. To establish the thermochemistry for the pyrolysis of cyclopentadienone, ab initio electronic structure calculations (AE-CCSD(T)/aug-cc-pCVQZ//AE-CCSD(T)/cc-pVQZ and anharmonic FC-CCSD(T)/ANO1 ZPEs) were used to find Delta H-f(0)(C5H4=O) to be 16 +/- 1 kcal mol(-1) and Delta H-f(0)(CE2=CH-C=CH) to be 71 +/- 1 kcal mol(-1). The calculations predict the reaction enthalpies Delta H-rxn(0)(1) to be 28 +/- 1 kcal mol(-1) (Delta H-rxn(298)(1) is 30 +/- kcal mol(-1)) and Delta H-rxn(0)(2) to be 66 +/- 1 kcal mol(-1) (Delta H-rxn(298)(2) is 69 +/- 1 kcal mol(-1)). Following pyrolysis of C5H4=0, photoionization mass spectrometry was used to measure the relative concentrations of HCC-CHCH2 and HCCH. Reaction 1 dominates at low pyrolysis temperatures (1000-1400 K). At temperatures above 1400 K, reaction 2 becomes the dominant channel. We have used the product branching ratios Over the temperature range 1000-1600 K to extract the ratios of unimolecular rate coefficients for reactions 1 and 2. If Arrhenius expressions are used, the difference of activation energies for reactions 1 and 2, E-2 - E-1, is found to be 16 +/- 1 kcal mol(-1) and the ratio of the pre-exponential factors, A(2)/A(1) is 7.0 +/- 0.3.