Hypercrosslinked polystyrene samples were prepared by an intensive postcrosslinking of highly swollen styrene-divinylbenzene copolymer beads to extremely high degrees of croslinking that amounted to 100% or even higher. When in the dry state, the materials obtained represent transparent beads of reduced density. Despite the high degree of crosslinking, the materials manifest large increases in volume on swelling with any liquid media as well as large changes in volume on heating. The factors determining the unusual swelling ability of the hypercrosslinked polymers are briefly discussed. Thermodilatometric tests of the polymers with a moderate degree of crosslinking reveal a certain contraction of the beads at, temperatures higher than 100 degrees C. When crosslinked far beyond 100%, the networks demonstrate unusual expansion in the 100-220 degrees C range; this is followed by a sharp shrinking at higher temperatures. The former is caused by an increased intensity of the vibration movements of the network and a partial relaxation of strong inner stresses; the latter is due to partial chemical oxidation, degradation of network units, or both. This degradation, however, is not accompanied by any loss in weight of the polymer but only results in a transformation into a more dense conventional nonporous material. The strained, rigid openwork structure of the homogeneous expanded hypercrosslinked polystyrene networks is responsible for their unusual mobility.