In this paper, a control strategy to reduce the vibrational response of adjacent buildings under seismic excitation is presented. The proposed strategy combines passive linking elements with an active decentralized H-infinity control system. The overall active-passive control system admits decentralized design and operation, and achieves an excellent vibrational reduction when the active control system works; in case of a full or partial failure of the active control system, a remarkable reduction in the vibrational response is guaranteed by the passive linking elements. For adjacent buildings that require different levels of seismic protection, the implementation of an active H-infinity control system in just one of the buildings is also considered. The main ideas are presented by means of a simplified two-building model. Numerical simulations have been carried out to assess the performance of the proposed methodology with promising results