Ab initio calculations are utilized to search for transition state structures for proton transfer in the 1,2,3-triazole-triazolium complexes on the basis of optimized dimers. The result suggest six transition state structures for single proton transfer in the complexes, most of which are coplanar. The energy barriers, between different stable and transition states structures with zero point energy (ZPE) corrections, show that proton transfer occurs at room temperature with coplanar configuration that has the lowest energy. The results clearly support that reorientation gives triazole flexibility for proton transfer. (C) 2010 Elsevier B.V. All rights reserved.