In this study we used the DFT B3LYP/6-311G(d,p) quantum mechanical method to propose a theoretical explanation of the regioselectivity observed experimentally in the reaction between trichloromethylphosphine oxide and triethyl phosphite using a variety of approaches,  notably frontier molecular orbital theory and calculation of activity energies and reactivity indices. Our results show that triethyl phosphite functions as a nucleophile while trichloromethylphosphine oxide behaves as an electrophile. The nucleophilic attack takes place preferentially at the chlorine atom of trichloromethylphosphine oxide rather than at the carbon, and the reaction is polar and regioselective. These results are comparable with those obtained experimentally.