There is disclosed a heat-resistant agent which exhibits good solubility in various organopolysiloxanes and which confers superior heat-resistance thereto, said heat-resistant agent having the formula ##STR1## wherein R.sup.2 is independently selected from hydrocarbon or halogenated hydrocarbon groups having 1 to 10 carbon atoms, R.sup.1 is independently selected from R.sup.2 or a eugenol residue, m is an integer having an average value of 5 to 500 and n is an integer having an average value of 0 to 500, with the proviso that there is at least one eugenol residue in the molecule. Also disclosed is a composition comprising an organopolysiloxane and the above described heat-resistant agent.