comprehensive B3LYP/6-31+G* study of various 1,5-electrocyclizations in nitrile ylides was performed. The complete reaction pathway for each reaction was determined, and changes in magnetic susceptibility and its anisotropy were monitored during the process. This allowed us to examine aromaticity changes and classify the reactions into two distinct groups. In one, the reaction involves a marked increase in aromaticity in the vicinity of the transition state, which can be ascribed to a typical pericyclic behavior; in the other, no such aromatization is observed, so the underlying mechanism must be one of the pseudopericyclic type involving an in-plane attack of the lone pair in the heteroatom.