Expanded polystyrene (EPS) geofoam has been used as lightweight subgrade for rigid pavement construction over weak soils. Design methods require modulus of subgrade reaction as input parameter for characterizing subgrade materials underlying rigid pavement structures. Designs of structural foundations over EPS geofoam also utilize modulus of subgrade reaction. Available information related to estimation or appropriate test methods for evaluation of modulus of subgrade reaction of EPS geofoams is very limited. A series of plate load and CBR tests were performed on different densities of geofoams. Effects of plate size and geofoam density were examined. Both plate load and CBR tests resulted in low design parameters for all geofoam grades. Plate load test results were used to calibrate a numerical model. Estimates of k values for concrete slabs over geofoams, assuming composite subgrades, were obtained from the calibrated numerical models. The equivalent k values for the combined concrete slab and geofoam subgrade layers were higher than for geofoam subgrades without concrete cover. The equivalent k values also indicate significant differences between low and high-density geofoam grades for the same thickness of concrete slab. Estimates of k for concrete slab and geofoam composite subgrades agree with previously reported assessments from field surveys and back calculations. Copyright ASCE 2006.