The electron localization function (ELF), a local measure of the Pauli repulsion, is shown like a useful descriptor of bonding at pericyclic and pseudopericyclic transition states. The main differences between these two relevant topologies have been investigated in detail through the examination of well-characterized typical allowed-symmetry thermal decarbonylations. It is shown that results based on the electron fluctuation between the ELF basin populations at the reaction center, provides a consistent description of bonding which complements the traditional Woodward-Hoffmann symmetry-orbital based analysis. (C) 2003 American Institute of Physics.