The Gough‐Joule coefficients (a) at constant stress and (b) at constant strain are defined and methods of measurement of each on vulcanized rubbers are given. It is shown mathematically and experimentally that the ratio of (b) to (a) is equal to the ``tangent'' modulus. The quotient of the Gough‐Joule coefficient at constant stress divided by the stress and the quotient of the Gough‐Joule coefficient at constant strain divided by the strain are shown to be constants which are independent of the stress‐strain‐temperature conditions of the test. Experimental data are given for stocks based on Hevea, GR‐S, Neoprene GN, Butyl Rubber, and Butaprene.