A new seismic isolation system for steel building structures based on the principle of mass isolation is introduced. In this system, isolating interfaces are placed between the lateral-load-resisting sub-system and the gravity-load-resisting sub-system. Because of the virtual decoupling existing between the two structural sub-systems, the gravity-load resisting one is susceptible to instability. Due to the fact that the provided level of isolation from the ground is constrained by the stability requirements of the gravity-load resisting structure, the system is named stability-dependent mass isolation (SDMI). Lyapunov stability and its association with energy principles are used to assess the stable limits of the SDMI system, its equilibrium positions, the stability of the equi-librium positions, and to propose a series of design guidelines and equations that allow the optimal seismic performance of the system while guaranteeing the restoration of its undistorted position. It is mathematically shown that the use of soft elastic interfaces, between the lateral- and gravity-load-resisting sub-systems, can serve the dual role of stability braces and isolators well. The second part of the document is concerned with the analytical evaluation of the seismic performance of the SDMI method. First, a genetic algorithm is used to find optimized SDMI building prototypes and, later, these prototypes are subjected to a series of earthquake records having different hazard levels. This analytical testing program shows that, with the use of SDMI, not only can structural failure be avoided, but a dam-age-free structural performance can also be achieved, accompanied by average reductions in the floor accelerations of ca. 70% when compared to those developed by typical braced-frame structures. Since the SDMI system is to be used in conjunction with viscous energy dissi-paters, the analytical testing program is also used to determine the best places to place the dampers so that they are most effective in minimizing the floor accelerations and controlling the floors’ drift-ratios. Finally, recommendations on continuing research are made.