The geometries of the reactant, product, and transition state of the reaction between acetylene and the hydrogen atom have been optimized at the level of HF/6-31G using the energy gradient method. The barrier height for this reaction was calculated to be 105.9 kJ/mol with a zero-point energy correction and elimination of spin contamination. The intrinsic reaction coordinate (IRC) for this reaction was traced and the coupling between the IRC and normal modes was analyzed along the IRC. The variational transition-state theory with the correction of the tunnel effect has been used to evaluate the reaction rate constant. It is in good agreement with an experimental estimate. © 1994 John Wiley & Sons, Inc.