Ab initio calculations were performed to investigate reaction mechanisms for formation and decomposition of the six-membered ring C3N3H3, known as sym-triazine. MP2 geometry optimizations with QCISD(T) energy refinements for critical points on the potential energy surface were calculated with the 6-31G** 6-311++G** and cc-pVTZ basis sets. Good agreement is found for MP2 geometries and frequencies of sym-triazine and HCN when compared with the corresponding experimental values. Two decomposition mechanisms of sym-triazine, the concerted triple dissociation (sym-triazine --> 3 HCN) and the stepwise decomposition (sym-triazine --> H2C2N2 + HCN --> 3 HCN) were investigated. All calculations show that the lowest energy decomposition mechanism is the concerted triple dissociation. Our best calculations predict the zero-point-energy-corrected barrier to decomposition to be 81.2 kcal/mol. The calculated reaction enthalpy is 35.5 kcal/mol, 7.7 kcal/mol lower than experiment. Intrinsic reaction coordinate calculations leading from the transition state of the concerted triple dissociation reaction to three HCN molecules led to a minimum on the potential energy surface. The corresponding structure is a cyclic (HCN)(3) cluster. The temperature-corrected formation enthalpy of the cluster is -8.7 kcal/mol relative to three isolated HCN molecules. The zero-point-corrected barrier to formation of sym-triazine from the cluster is 58.1 kcal/mol. QCISD(T) energy refinements did not differ significantly from the MP2 results.