Photoemission from the following excited species was observed from OCS in a helium afterglow at wavelengths from 200 to 500 nm: OCS + ( Ã 2Π- X̃ 2Π), CO( a 3Π- X 1Σ +), CO +( A 2Π i - X 2Σ +, B 2Σ +- X 2Σ +), CS( A 1Π- X 1Σ +, a 3Π- X 1Σ +) and CS +( B 2Σ +- A 2Π i ). The dependence of emission intensities on the electrostatic potential applied to an ion-collector grid indicated that OCS +( Ã- X̃) emission results from He( 2 3 S) Penning ionization, while CO +( A- X, B- X) emission is the result of dissociative charge-transfer ionization of OCS with He + ions. The emission rate constants were estimated to be 5.1 ± 0.5 × 10 −13 cm 3 molecule −1 s −1 for OCS +( Ã) and about 9 × 10 −12 cm 3 molecule −1 s −1 for CO + ( A). Energy resonance was found to be an important factor for the thermal He +/OCS reaction.