Experimental evidence is presented to show that when cross-linked natural rubber samples are heated in air whilst subject to a pure shear strain and subsequently freed of all forces the new force-free or “set” state is not related to the original force-free state by a pure shear. During a pure shear linear elements along one of the principal directions of strain remain unchanged in length; it is shown that these elements undergo an increase in length as a result of the recovery after heating. The results are compared with the predictions of the “two-network” theory of permanent set propounded by previous workers.