Potential energy surfaces for the various mechanisms of the dissociation of C6H6 producing different isomers of the C5H3 radical have been investigated using the ab initio modified Gaussian-2 (G2M) method. The most stable structures of C5H3, H2CCCCCH (II1) and HCCC(H)CCH (II2), can be formed from benzene and fulvene through the pathways involving a 1,2-H shift in the carbon ring and the ring opening followed by the series of hydrogen shifts in the open chain structures of C6H6. The reaction is completed by the elimination of CH3 from C5H3-CH3. All the transition states along the reaction pathways are found to lie below similar to 143 kcal/mol relative to benzene, while the heats of the reactions, benzene --> H2CCCCCH + CH3 and benzene --> HCCC(H)CCH + CH3, are calculated to be about 150 kcal/mol. On this basis, the C5H3 + CH3 product channel of the photofragmentation of benzene at 248 nm is expected to be a two-photon process. In total, 25 different isomers of C5H3 have been calculated. The most stable, II1 and II2, are predicted to have the enthalpies of formation of 136.5 and 137.2 kcal/mol, respectively. Contrary to C5H3+, the ethynyl cyclopropenyl isomer of neutral C5H3 is only the third in stability with a Delta H-f(298) of 166.9 kcal/mol.