The intrinsic reaction coordinate (IRC) is a minimum-energy pathway connecting reactants to products via the transition state. An improved algorithm for the determination of an IRC is presented. The method is illustrated for the rotation of the silicon-carbon double bond in silaethylene. This IRC shows all coordinates vary smoothly during the rotation from the planar to twisted structures, except for a slight pyramidalization at carbon. The rotational barrier is found to be about 37 kcaljmol, in good agreement with experimental estimates.