The electrophilic addition of hydrogen chloride in substituted ethylenes has been revised by using the local hardness as a descriptor of selectivity. The regioselectivity shown by the local hardness values is in agreement with the empirical Markovnikov's rule. The effects of the electron-donating and electron-withdrawing substituents in the addition reactions have been studied in detail by calculating the energy of addition reaction and the activation energy for the transition states. It has been found that the strong electron-withdrawing substituents prefer to lead anti-Markovnikov products, while the electron-donating and weak electron-withdrawing groups prefer to form Markovnikov products. (C) 2007 Elsevier B.V. All rights reserved.