Abstract The oscillator strength was calculated for the vinyl radical at MRCISD and CASSCF levels between the ground state and several doublet excited states. Results indicated that the C ̃ 2 A ′← X ̃ 2 A ′ transition, of π *←π character that was usually thought to be strong, was very weak. The C ̃ 2 A ′ state is the result of coupling between the π *←π triplet excitation and the nonbonding electron, and the transition is essentially ‘spin-forbidden’. Another high-energy doublet state was found with strong transition from the ground state arising from coupling between the π *←π singlet excitation and the nonbonding electron. This appears to be a common feature for π *←π excitation in σ radicals.