A deterioration of physical properties that may occur upon excessive vulcanization of some elastomers, evidenced by a decrease in hardness and tensile strength, and an increase in elonga- tion; (2) A similar change in proper- ties after air aging at elevated temperatures. Natural rubber, butyl, polysulfides and epichlorobydrin polymers exhibit this effect (extreme reversion may result in tackiness). Most other polymers will harden and suffer loss of elongation on hot air aging. The softening of some vulcanized rubbers when they are heated too long. Usually accompanied by an increase in extensibility, a decrease in tensile strength and a lowering of the stress required to produce a given elongation. Extreme reversion may result in tackiness; the rubbers “revert” to an unvulcanized then to a non-polymeric condition.