A relatively new approach for shock and vibration isolation is to use semi-active or fully-active elements, which are designed to provide better vibration isolation over a broad range of excitation inputs. Several vibration isolation strategies have been proposed and developed using variable damping elements, variable stiffness elements and different actuators. An interesting, novel control strategy is to have an isolator whose stiffness can take two values in an on-off manner. In this paper such a system is described and modelled, and in particular the free vibration decay behaviour is quantified and discussed. This paper also describes the benefits of the variable stiffness isolator and quantifies the effective damping in terms of simple model parameters.