Protecting critical equipment in seismically excited buildings is essential for maintaining their functionality after an earthquake. One approach for protecting equipment is through isolation, where individual pieces of equipment, or equipment sets, are isolated from the building motion. Because critical equipment often contains acceleration-sensitive components that can be damaged during an earthquake, it is necessary to limit the inertial forces experienced by the equipment during an event. This is particularly challenging on the upper floors of buildings where dynamic amplification of the ground acceleration is significant. Protecting equipment on the upper floors of buildings can be achieved using special isolators with restoring forces that can be shaped according to the isolator displacements. The objective of this research is to investigate an adaptive restoring force (ARF) device that can be used for the horizontal isolation of acceleration-sensitive equipment. The force-displacement relationship of the ARF device is characterized by high stiffness at small and large isolator displacements and low stiffness in between. It utilizes cables to transfer motion and forces between the isolation platform and the device components. The force-displacement characteristics of the ARF device can be controlled through the proper selection of its component geometry to produce a wide range of isolator responses. Numerical analyses were performed to evaluate the effectiveness of the device for horizontal seismic isolation of lightweight equipment installed at various levels of a multistory building. The results suggest that isolation systems using ARF devices are capable of significantly reducing floor accelerations transmitted to floor-mounted equipment. However, this often comes at the cost of large isolator displacements.