A new potential energy surface for C2H2 that describes acetylene/vinylidene isomerization is reported. The surface is an accurate, least-squares fit to nearly 10,000 symmetry-equivalent, ab initio electronic calculations obtained at the CCSD(T) level of theory, with an aug-cc-pVTZ basis. The ab initio geometries and normal-mode frequencies of the acetylene and vinylidene minima, and saddle point are reproduced very well by the fitted potential energy surface. Full-dimensional calculations of low-lying acetylene vibrational energies are also reported and compared to experiment. (C) 2002 Elsevier Science B.V. All rights reserved.