Ellipticity changes of the electron density at the bond critical points along the [2 + 2] cycloaddition reaction paths were investigated by the AIM method applied to the MP2/aug-cc-pVDZ calculated structures. Elipticities of the sigma-bonds being formed and pi-bonds being broken in the ketene-vinylimine and allene-isocyanic acid cycloadditions were examined. On this basis we proposed four criteria for differentiation between pericyclic and pseudopericyclic reactions and classified the ketene-vinylimine reaction as pseudopericyclic whereas the allene-isocyanic acid one as pericyclic process. Additionally, we found the pi(C=X) ellipticity profiles, X=N, O, suggesting more or less isotropicity of the C=X bond electron density over the whole reaction. (c) 2007 Elsevier B.V. All rights reserved.