In this study, the equivalent linear (EL) analysis procedure used in the design of seismic-isolated structures is evaluated. The effect of several parameters such as the intensity and frequency characteristics of the ground motion, isolator properties and the structure mass are considered in the evaluation. First, the EL analysis procedure is evaluated using harmonic ground motions with various excitation periods and a suite of 15 seismic ground motions with various frequency characteristics. Then, the effect of the structure mass on the effective damping ratio and hence on the accuracy of the EL analysis results is investigated. This is followed by regression analyses of the acquired data to assess the effect of various parameters on the accuracy of the EL analysis results. It is found that the accuracy of the EL analysis results is affected by the peak ground acceleration to peak ground velocity ratio of the ground motion as well as the intensity of the ground motion relative to the characteristic strength of the isolator. It is also demonstrated that the effective damping equation currently used in the design of seismic-isolated structures must incorporate the effective period of the structure and frequency characteristics of the ground motion for a more accurate estimation of seismic response quantities. A new effective damping equation that includes such parameters is formulated and found to improve the accuracy of the EL analysis results. (c) 2006 Elsevier Ltd. All rights reserved.