For earthquake excitation, base isolators have been successfully used in building structures on firm ground. However, the use of these devices in buildings on soft soil for SF6 gas-insulated substations introduces the following concerns: a) the frequency and damping characteristics of the fixed-base structure may change significantly by the combined effects of soil-structure interaction (SSI) and the base isolation device; and b) very large displacements may result because of the translation and rocking of the foundation. In this paper, the effects of SSI on the seismic response of a base-isolated MDOF system are investigated. Both kine-matic and inertial effects are considered. The effective input motions (translation and rocking) for an embedded foundation are computed with the averaging Iguchi's method. The soil is replaced by frequency-dependent elastic springs and viscous dampers computed with a finite layer method. The SSI analysis is performed in the frequency domain with the complex fre-quency method. Numerical results, including floor response spectra of acceleration and dis-placement, are computed for SSI conditions prevailing in Mexico City, where SF6 gas-insulated substations are being projected. The base isolation device must be capable of ac-commodating displacements associated with different performance levels.