A simple, unique scale based on the analysis of electron localization function (ELF) is introduced in order to categorize the degree of pericyclicity of a given electrocyclic concerted transition state. The nature of pericyclic/pseudopericyclic bonding at such type transition states becomes thus described in terms of a local measure of the Pauli repulsion. The nature of bonding of ten cycloaddition transition states associated to the reaction between ethylene and formaldehyde and buta-1,3-dien-one and simple substituted derivatives, has been discussed in this context. (c) 2007 Elsevier B.V. All rights reserved.