Singlet-oxygen [O2((1)Delta(g))] generation by valence-excited thiophene (TPH) has been investigated using multireference Moller-Plesset second-order perturbation (MRMP2) theory of geometries optimized at the complete active space self-consistent field (CASSCF) theory level. Our results indicate that triplet TPH(1(3)B(2)) is produced via photoinduced singlet TPH(2(1)A(1)) because 2(1)A(1) TPH shows a large spin-orbit coupling constant with the first triplet excited state (1(3)B(2)). The relaxed TPH in the 1(3)B(2) state can form an exciplex with O-2(((3)Sigma(-)(g)) because this exciplex is energetically more stable than the relaxed TPH. The formation of the TPH(1(3)B(2)) exciplex with O-2((3)Sigma(-)(g)) whose total spin multiplicity is triplet T-1 state) increases the likelihood of transition from the T-1 state to the singlet ground or first excited singlet state. After the transition, O-2((1)Delta(g)) is emitted easily although the favorable product is that from a 2 + 4 cycloaddition reaction.