According to the local hard-soft acidbase (HSAB) principle, the generalized local softness has been investigated based on the electrophilic reactions of H 2 O, HF, HCl and HBr to alkenes, and the effect of the softness on the chemical reactivity was studied in terms of the ab initio and atom-bond electronegativity equalization method plus σπ (ABEEM-σπ) method. Studies show that the finite difference approximation with ab initio method cannot always be used to predict the regioselectivity of the investigated reactions in terms of the local HSAB principle. But ABEEM-σπ model could well describe chemical reactivity according to the local HSAB principle. A significant characteristic of the ABEEM-σπ method is that the double bond is explicitly considered and partitioned. The square sums of differences of the local softness of the reaction center atoms, that is, Δs, or ΔS G with the generalized local softness and Δs χ with the electronegativity, have been studied on the considered electrophilic additions. Studies indicate that ΔS G can be utilized not only to predict regioselectivity rules but also to rationalize the chemical reactivity of these addition reactions, and the generalized local softness is more suitable for the investigation of chemical reactivity between two chemical species.