Reactive scattering of ground state O(P-3) atoms with C2H4S and C4H4S molecules has been studied at an initial translational energy E similar to 40 kJ mol(-1) using a supersonic beam of O atoms seeded in He buffer gas generated from a microwave discharge source. The center-of-mass angular distribution of SO scattering for O + C2H4S is cone-shaped in the backward hemisphere at a scattering angle a theta similar to 120 degrees with respect to the incident O atom direction, while the OC4H3S scattering for O + C4H4S is nominally isotropic. The O + C2H4S reaction disposes a lower fraction (f’ similar to 0.3) of the total available energy into product translation than the O + C4H4S reaction (f’ similar to 0.4), although the product translational energies are higher by a factor of similar to 2 for the more exoergic O + C2H4S reaction. The O + C2H4S reaction involves cleavage of the three-membered ring of the thiirane molecule, while the O + C4H4S reaction involves H atom displacement from the five-membered ring of the thiophene molecule. Both reactions occur on the triplet potential energy surface. The strongly exoergic concerted rupture of the three-membered thiirane ring disposes half the available energy into vibrational excitation of the ethene product molecule compared with only a fraction (similar to 0.13) being disposed into SO product vibration. The displacement reaction of the five-membered thiophene ring involves O atom addition forming a persistent OC4H4S complex with a potential energy barrier to H atom displacement.