The data obtained in testing expanded polystyrene (EPS) slabs subjected to short-term compression are discussed. The results demonstrated that the compressive strength of the slabs should be determined according to critical stress taking into account the type of deformations shown in the stress-strain diagram. The regression equations are provided for calculating the compressive stresses at 10 % relative deformation and for critical stress of EPS slabs with the density ranging from 12 to 44 kg/m3.