It is well known that through thermal energy storage (TES) systems it is possible to store (release) thermal energy by heating (cooling) a medium in order to utilize the stored energy when required. Possible applications involve power generation systems [1][2][3][4] and building constructions [5], where about one half of the energy demand is in the form of thermal energy, and the requirement may markedly vary in time [6][7][8]. TES technology have been also investigated in the textiles industry for the production of 'smart' fabrics able to maintain the right temperature of the body [9][10][11][12][13][14]. Other applications could be represented by the food storage [15,16] and the development of innovative solar plants [17][18][19]. Latent heat TES systems has recently attracted the attention of researchers, because these systems are characterized by a high energy storage density at constant temperature corresponding to the transition temperature of the phase change material (PCM) [18,20]. Generally speaking, a solid/liquid or a solid/solid phase transition could occur, and depending on their chemical nature these materials can be classified as organic, inorganic or eutectic. Organic PCM have several advantages [21,22] Abstract. The aim of this paper is to develop new elastomeric phase change materials (PCM) for the thermal energy storage/release below room temperature. In particular, poly(cyclooctene) (PCO)/paraffin blends filled with various concentrations of carbon nanotubes (CNTs), were prepared by a melt compounding process. The microstructural, thermo-mechanical and electrical properties of the resulting materials were investigated. The microstructure of these materials was characterized by the presence of paraffin domains inside the PCO, and CNTs were located only inside the paraffin domains in forms of aggregated clusters. DSC tests evidenced the existence of two distinct crystallization peaks at -10 and at 6°C, respectively associated to the paraffin and the PCO phases, indicating that both the polymeric constituents are thermally active below room temperature. Moreover, CNT addition did not substantially alter the melting/crystallization properties of the material. Noticeable improvements of the mechanical properties and of the electrical conductivity with respect to the neat PCO/paraffin blend could be obtained upon CNT addition, and also thermal conductivity/diffusivity values were considerably enhanced above the percolation threshold. Finite element modeling demonstrated the efficacy of the prepared nanocomposites for applications in the thermal range from -30 to 6°C.