A study is made of the seismic behaviour of tuned equipment–structure systems where one or both of the system components experience inelastic deformation. The response is determined using coupled and decoupled models of the system. The effect of various parameters such as mass ratio and yield level on the system response is evaluated.It was found that the mass ratio and yield level of the tuned inelastic system are the key parameters affecting the response of the coupled system. The response of the primary system is found to be insensitive to the variation of the yield level of the secondary system. In addition, the response obtained using uncoupled analysis of the equipment and structure system is generally higher than the response obtained using coupled analysis and may result in grossly overdesigned systems. Key words: dynamic, seismic, response, tuned, coupled, equipment, structure, earthquake, inelastic.