SUMMARYThis paper concerns the design of passive base isolation systems characterized by a bilinear hysteretic behaviour. The study refers to the case where the structure to be isolated (superstructure) vibrates according to the ÿrst mode. In this case the whole isolated structure can be modelled by a two-degreeof-freedom system. The base isolation e ectiveness has been evaluated for di erent characteristics of the device, namely mass, strength, elastic and plastic sti ness, by using mainly energetic quantities.The optimum values for the base device have been obtained by minimizing the input energy and the displacement of the superstructure. Conclusions are drawn for superstructures with a fundamental period of 0:5 s, a damping ratio of 5% and for three di erent kinds of earthquake ground motions. The study showed that the seismic input greatly a ects the behaviour of the isolated structure, and therefore the design ground motion must be carefully chosen, dependent on the characteristics of the site. A simple procedure that involves mainly linear dynamic analyses is proposed for the design of base devices used in conjunction with superstructures of any fundamental vibration period. The procedure produces good results in spite of its simplicity, and therefore it is suitable for practical use by design engineers.