Information concerning the state or degree of vulcanization of rubber products is often desired. This is because most products are compounded and cured to a specific hardness or modulus depending on the end use. For example, stocks used in mechanical goods usually have a higher hardness than goods where fatigue resistance is important, such as tires. If the modulus changes, however, during the life of the article, this is usually reflected in a shortened service period. For general-purpose diene rubbers, this change depends on the type of polymer: polyisoprene rubbers such as natural rubber tend to soften slightly on aging or service, whereas rubbers derived from butadiene such as styrene-butadiene copolymer tend to harden considerably. Typical physical test methods are usually out of the question. The determination of crosslink density by equilibrium swelling measurements is the only practical method that is presently available. Moreover, any size or shape of sample may be used, which makes the method adaptable to routine measurements for new or aged products.