The mechanism for the C(P-3) + C2H3 reaction has been studied via ab initio calculations to investigate possible formations of C3H2 and C3H isomers in an extraterrestrial environment, combustion processes, and CVD. The C(P-3) + C2H3 reaction, which produces C3H3 radical intermediates on the ground-state potential energy surface (PES), is studied employing the B3LYP/6-311G(d,p) and RCCSD(T)/6-311+G(3df,2p) levels of theory. Initially formed C3H3 intermediates have enough energy to undergo unimolecular rearrangements. Further, H or Hz eliminations then lead to C3H2 Or C3H fragments. The most energetically favorable channel is found to be the formation of ground-state singlet cyclopropenylidene (c-C3H2, (1)A(1)) by splitting H from cycloprop-2-enyl (C-C3H3, (2)A '). The other reaction mechanisms leading to H2CCC((1)A(1)) + H, HCCCH(B-3) + H, and H-2 + HCCC(2 Pi) exhibit barriers only 1-5 kcal/mol higher than those to produce H + C-C3H2. Detailed RRKTW calculations will be needed to predict the product branching ratios under various reaction conditions. The C(P-3) + C2H3 reaction channel, yielding intermediate C3H3 radicals on the first excited doublet state PES, is also studied by utilizing the CASSCF(11,11)/6-311+G(d,p) and MRCI + D(7,8)/ANO(2+) levels of theory. Three local minima and six transition states are located on the excited-state C3H3 PES. Various H and H: loss channels are studied as well. The C-H fission of H2CCCH(1(2)A ") leading to HCCCH(1(1)A ") + H is the most energetically favorable channel. Finally, thermochemical parameters for the C3Hn (n = 1-4) species are determined by employing the G3 theory and the CCSD(T)6-311 + G(3df,2p) method. The differences between the calculated results and available literature data do not normally exceed 1-2 kcal/mol. On the basis of the present calculations and previous theoretical and experimental data, DeltaH(f)(298)(o)(H2CCCH) = 84.5 +/- 1 kcal/mol, DeltaH(f)(298)(o)(c-C3H3) = 114.5 +/- 2 kcal/mol, DeltaH(f)(298)(o)(H3CCC) = 124 +/- 2 kcal/mol, DeltaH(f)(298)(o)(c-C3H2) = 118.0 +/- 1 kcal/mol, DeltaH(f)(298)(o)(H2CCC)= 133 +/- 1 kcal/mol, DeltaH(f)(298)(o)[HCCCH(B-3)] = 132.5 +/- 1 kcal/mol, DeltaH(f)(298)(o)[HCCCH(B-3)] = 144 +/- 1 kcal/mol, DeltaH(f)(298)(o)[HCCC((2)Pi)] = 173 +/- 2 kcal/mol, and DeltaH(f)(298)(o)(C-C3H) = 170 +/- 2 kcal/mol are recommended.