Recent theoretical developments that facilitate characterization of excited-state potential energy surfaces are applied to study five stationary points on the S-1 surface of C2H2. Although previous calculations have focused on the acetylenic trans and cis forms, the present research predicts that the vinylidene isomer is the global minimum on the potential surface. However, a high activation barrier calculated for rearrangement to the trans isomer suggests that S-1 vinylidene is not likely to be formed via photoexcitation of ground-state acetylene. The trans and cis acetylenic forms of the S-1 state are shown to interconvert along an in-plane reaction coordinate with an activation energy of similar to 4000 cm(-1), a value which is significantly lower than usually assumed in spectroscopic analyses.