Ab initio multiconfigurational CASSCF and CASPT2 methods were employed in studying the reaction mechanisms and kinetics of the gas-phase ozone additions to ethene, fluoroethene, and chloroethene up to the formation of the primary addition products (primary ozonides). Reactants, transition-state structures, and products were optimized, and harmonic vibrational frequencies were calculated at the CASSCF/cc-pVTZ level. For kinetic calculations, the electron energies of all the stationary points were further refined by utilizing the CASPT2 method with the optimized CASSCF/cc-pVTZ wave functions taken as the zeroth order. The rate constants and Arrhenius kinetic parameters were finally calculated in terms of the conventional transition-state theory. The favorable conformations of the ozone approach to the two asymmetrically substituted haloalkenes are at first governed by the electrostatic repulsion in the transition-state structures and later by the gradually predominating anomeric effect. The bond formation in the primary haloozonides was analyzed by monitoring the changes in the occupation numbers of the active orbitals in the course of the optimizations. For all the reactions thus studied, close agreement is found with the experimental kinetics, which makes the future use of the same approach very promising.