We have investigated ab initio the existence of localized states and resonances in abrupt GaAs∕Si∕GaAs (110)- and (100)-oriented heterostructures incorporating one or two monolayers (MLs) of Si, as well as in the fully developed Si∕GaAs (110) heterojunction. In (100)-oriented structures, we find both valence- and conduction-band related near-band edge states localized at the Si∕GaAs interface. In the (110) systems, instead, interface states occur deeper in the valence band, the highest valence-related resonances being about 1 eV below the GaAs valence-band maximum. Using their characteristic bonding properties and atomic characters, we are able to follow the evolution of the localized states and resonances from the fully developed Si∕GaAs binary junction to the ternary GaAs∕Si∕GaAs (110) systems incorporating two or one ML(s) of Si. This approach also allows us to show the link between the interface states of the (110) and (100) systems. Finally, the conditions for the existence of localized states at the Si∕GaAs (110) interface are discussed based on a Koster-Slater model developed for the interface-state problem.