Abstract: A theoretical study of the "walk" rearrangement in bicyclo[2.1.0]pentene and perfluorotetramethyl (Dewar thiophene) exo-S-oxide has been carried out. In order to analyze the pericyclic character of these two reactions, some magnetic properties (NICS and ACID) have been calculated in the reactant/product and in the transition state of each reaction. Despite the differences between them, the results for both reactions show an enhancement of aromaticity in the transition state, which is consistent with a pericyclic behavior. NBO calculations show that the small activation energy for the second reaction can be interpreted in terms of a strongly stabilization of the transition state by the exo-oxide substituent. No evidence of any pseudopericyclic character has been found. Although the walk rearrangement in erfluorotetramethyl (Dewar thiophene) exo-S-oxide has special characteristics, the process of [1,3]-sigmatropic shift remains with a fundamental role in its mechanism. So, the mechanism proposed by Lemal et al, should be revised.