The mechanism of photodissociation of the vinyl radical, C2H3, starting from the first doublet excited (D-1,A) state was studied with high-level ab initio methods as well as with ab initio direct dynamics. Geometry optimizations of stationary points and surface crossing seams were performed with complete active space self-consistent field (CASSCF) method, and the energies were re-evaluated with single-point multireference single and double excitation configuration interaction (MRCISD) calculations. Both internal conversion and intersystem crossing channels, which could bring the excited vinyl radical down to the ground state potential energy surface leading to dissociation on the ground state, have been identified within planar C-s, twisted C-s and C-2v symmetry. Direct dynamics calculation indicates that the most feasible reaction channel is the direct internal conversion from D-1 to the ground state (D-0) within planar C-s symmetry, through a minimum of seam of crossing (conical intersection) at an energy of about 80 kcal/mol (with respect to the ground-state equilibrium geometry). The other internal conversions from D-1 to D-0 through conical intersections within twisted C-s symmetry require energies of about 80 and 75 kcal/mol at the two minima of seam of crossing, respectively, and they are not favored dynamically without initial out-of-plane vibrational excitation. The intersystem crossing channels between D-1 and the lowest quartet state (Q(1)) and D-0 and Q(1) within twisted C-s and C-2v symmetry are not efficient due to the high energy of the minima of seam of crossing as well as the small spin-orbit coupling. (C) 2003 American Institute of Physics.