Abstract: Thermally crosslinked polysulfone may be made from linear polysulfone, such as polyethersulfone, in powder form blended with a powdered inorganic peroxide such as magnesium peroxide or another oxygen source, to form a mixture followed by compression inside a mold. The mixture is cured at a an elevated temperature, for instance above 325° C., for an effective period of time to form a dense object. The object is then boiled in water, optionally under pressure, to remove the salt to give a structure that is open and porous which may be used as a filtration device on a downhole tool for hydrocarbon recovery. If a powdered salt is not used, a thermally crosslinked, solid, void-free polysulfone is made which may be strong and rigid at ambient, surface temperatures, but is an elastomer at elevated downhole temperatures, and is thus suitable for use as a packer or an O-ring.