Mechanisms of four 1,2-addition reactions, H2Si=SiHMe + H2O, H2Si=SiHF + H2O, H2Si=SiH(C≡CH) + H2O, H2Si=SiH(NH2) + HF, were investigated in detail by the ab initio MO method using a recent approach combined with frontier MO theory. Twelve reaction pathways were found. The initial step of each reaction is the formation of a weakly bonded complex. According to the structure and the charge distribution of the complexes, the reactions are categorized into two types. Reactions starting from electrophilic interaction between the LUMO of water (or hydrogen fluoride) and the HOMO of disilene always result in syn-adducts. On the other hand, nucleophilic interaction between the HOMO of the water and the LUMO of disilene leads both syn and anti adducts. Depending on the acidity of the reagent and the charge on the silicon to be attacked, the reactions proceed in simple one-step mechanisms or in two-step ones via a Lewis-type complex. The paired interacting orbitals of the initial complexes were examined in order to predict the reaction pathways.