This article reports on the morphology, melting and crystallization behavior, thermal stability, tensile properties, and thermal conductivity of phase-change materials (PCM) for thermal energy storage. These materials were based on a soft Fischer-Tropsch paraffin wax (PCM) blended with low-density polyethylene, linear low-density polyethylene, and high-density polyethylene. These immiscible blends were melt-mixed with copper (Cu) microparticles (up to 15 vol %) to improve the thermal conductivity in the matrix material. The presence of the Cu microparticles in the PCMs did not significantly change the crystallization behavior, thermal stability, or tensile properties of the blend composites in comparison with the corresponding poly-ethylene/wax blends and polyethylene/Cu composites. The observed differences were related to the fact that the wax seemed to have a higher affinity for the Cu particles than any of the polyethylenes, and so it crystallized as a layer around the Cu particles. The thermal conductivity of the samples increased almost linearly with increasing Cu content, but the samples had slightly lower values than the corresponding polyethylene/Cu composites, probably because of the lower thermal conductivity of the wax. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1766-1774,2010