Abstract In the present theoretical work we have explored mechanisms of dissociation reactions of the vinyl radical in the A 2 A ″ state (C2H3 ( A 2 A ″ )) and examined possible pathways for nonadiabatic dissociation of C2H3 ( A 2 A ″ ) into C2H2 ( X 1 Σ g + ). In the calculations we used the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with the cc-pVDZ and cc-pVTZ basis sets. Mechanisms for the following three dissociation channels of C2H3 in the A 2 A ″ state were explored: (1) C2H3 ( A 2 A ″ ) → C2H2 (trans, 3 A u ) + H, (2) C2H3 ( A 2 A ″ ) → C2H2 (cis, 3 A 2 ) + H, and (3) C2H3 ( A 2 A ″ ) → H2CC ( 3 A 2 ) + H. The CASSCF and CASPT2 potential energy curve calculations for the C2H3 ( A 2 A ″ ) dissociation channels (1)–(3) indicate that there is neither transition state nor intermediate for each of the channels. At the CASPT2//CASSCF/cc-pVTZ level, the dissociation energies for channels (1)–(3) are predicted to be 84.3, 91.1, and 86.9 kcal/mol, respectively. For a recently observed nonadiabatic dissociation of C2H3 ( A 2 A ″ ) into C2H2 ( X 1 Σ g + ) + H [J. Chem. Phys. 111 (1999) 3783], two previously suggested internal conversion (IC) pathways were examined based on our CASSCF and CASPT2 calculations. Our preliminary CASSCF and CASPT2 calculations indicate that the assumed IC pathway via the twisted C2H3 ( A 2 A ′ ) structure might be feasible. The CASSCF/cc-pVTZ geometry optimization and frequency analysis calculations were performed for the four C2v bridge structures in the 2 B 2 , 2 A 2 , 2 B 1 , and 2 A 1 states along the pathways of the 1 2 A ′ ( X 2 A ′ ), 1 2 A ″ ( A 2 A ″ ), 2 2 A ″ , and 2 2 A ′ states of C2H3, respectively, and the CASPT2//CASSCF/cc-pVTZ energetic results indicate that the assumed IC pathway, via a C2v ( 2 A 2 ) structure and then 2 A 2 / 2 A 1 surface crossing, be not feasible since at their excitation wavelengths (327.4 and 366.2 nm) the C2v ( 2 A 2 ) structure could not be accessed.