Ab initio computations which included effects of electron correlation were performed at various N-N distances for the planar and the skew structure of N2O4 The results prove definitely the σ character of the bond electron pair. An investigation of the wave function by means of contour diagrams and a population analysis show that the very long N-N bond in N2C4 is due to the delocalization of the bond electron pair over the whole molecule and to a rather large repulsion between the doubly occupied MO's of the NO2 fragments. The coplanarity of N2O4 results from a delicate balance of the repulsive forces, which favor the skew structure, and of the effects of bonding which favor the planar structure.