A mechanism for the reaction of the Nb radical with vinyl chloride is proposed on the basis of B3LYP and CASSCF quantum chemistry calculations. Two initial transition states lead to the Markownikoff and contra-Markownikoff addition of NO3 to chloroethene. From: the two initial adducts different interconnected pathways start. We have found epoxichloroethane, chloroethanal, nitrous acid, acetyl chloride, nitric acid, formaldehyde, formyl chloride, and NO radical as main products.,The energy of the reactants, products,nd all intermediates along with the barrier heights for each reaction path has been calculated. The: geometry optimization and characterization of all of the stationary points found on the potential energy hypersurface was made at the B3LYP level with a 6-31G* basis set, and some of the stationary points have been reoptimized with the 6-311+G* basis set. Later, single-point CASSCF calculations were carried out with the same basis set, building the active; space with Seven active electrons in eight active orbitals. It can be concluded that an equilibrium distribution of formyl chloride, :formaldehyde; and the NO radical will:,be obtained along with small proportions of epoxychloroetane and the Markownikoff radical adduct. An estimation of the reaction rate constant has also been calculated.