The effect of flame retardants such as chloroparaffin, antimony trioxide, mixtures of antimony trioxide and chloroparaffin, melamine cyanurate and pentabromotoluene on the thermal properties of cis-1,4-polyisoprene peroxide vulcanizate has been studied. The thermoanalytical curves of elastomers were interpreted. The flame retardants used, except Sb2O3 were found to change the characteristic thermal transitions of polyisoprene and to cause a decrease in its thermal stability. From among the investigated flame retardants only pentabromotoluene favoured the degradation of the elastomer in the range of its first exothermic transition.