A new idea of molecular switching is presented, The idea is based on the intriguing phenomenon of complete reflection, which occurs in a two-state potential curve crossing of the nonadiabatic tunneling type. Complete switching of transmission is theoretically possible in one-dimensional systems by introducing impurities in the system. The basic semiclassical theory is presented, and the phenomenon of complete reflection is clearly interpreted and numerically demonstrated, An idea of an energy filter to facilitate the switching efficiently is also introduced. The possibility of bound states in the continuum is also clarified, This new molecular switching is numerically realized by the wave packet propagation. (C) 1997 American Institute of Physics.