Observation of mass-dependent and non-mass-dependent sulfur isotope fractionations in elemental sulfur is providing new insight into the nature of the sulfur cycle in the atmosphere. Interpretation of the experimental isotope data requires estimation of the energetics for the reaction S+S-2-> S-3 (isoelectronic with O+O-2-> O-3). Key molecular properties of the S-3 potential-energy surface, such as vibrational frequencies and isotopic shifts, are presented that can be used to assess the mass-dependent fractionation effect. Ab initio results are compared to the available experimental results for S-2 to evaluate the reliability of the computational results for S-3. The S-S bond dissociation energy for S-3 is determined to be 60.9 +/- 1 kcal mol(-1). (C) 2005 American Institute of Physics.