A new function, called zero steric potential (ZSP(r) = |∇ρ(r)|(2) - 2ρ(r)∇(2)ρ(r)), is proposed from the framework of density functional theory (DFT) to describe the regioselectivity of some selected reactions, including Diels-Alder reactions, addition of acids to alkenes, and Paternò-Büchi reactions. The ability of atomic zero steric potential (AZSP), which is obtained by integration of ZSP over an atomic basin, in predicting the major products of these reactions is checked. It is shown that for each reaction the least AZSP difference is observed for those atoms for which their binding leads to the main product. Therefore, it seems that the AZSP index could be used for predicting the reactive sites of reactants in a given chemical reaction.