The report examines the problem of establishing values of properties of seismic isolation bearings in the analysis and design of seismically isolated bridges. These bounding values of properties are determined with the use of system property modification factors. The property modification factors account for the effects of history of loading, the environmental conditions, and aging on the properties of the isolation bearings. The procedure requires that the mechanical behavior of the bearings be understood at both the macroscopic and microscopic levels. The report focuses on the effects of aging in elastomeric bearings and sliding bearings. The nature of friction in sliding bearings is also discussed. Property modification factors are presented for the effects of aging, contamination, travel, temperature, and scragging for selected interfaces and elastomeric bearings. The values represent the basis on which the bounding analysis is described in the 1999 AASHTO Guide Specifications for Seismic Isolation Design. (Adapted from authors" abstract).