Global warming is one of the most important consequences of excess energy consumption. Phase change materials (PCMs) have prominent advantages in thermal energy storage owing to their high latent heat capacities and small temperature variations during the phase change process. However, leakage is a major problem that limits the use of PCMs. Leakage may occur in encapsulated PCMs or in composites where the PCM is attached to the surface of a supporting material or within the pores of that material. In this study, pentadecane/diatomite and pentadecane/sepiolite nanocomposites were fabricated by using unmodified and microwave-irradiated diatomite and sepiolite samples and by using different compounding processes, such as direct impregnation, vacuum impregnation, and ultrasonic-assisted impregnation methods. The microstructures and the chemical and thermal properties of the composites were characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, and differential scanning calorimetry. Subsequently, the thermal reliability and stability and the thermal conductivity of the PCM composites were also investigated. A melting temperature of 9.25 degrees C and a latent heat capacity of 58.73 J/g were determined for the pentadecane/diatomite composite that was prepared with the direct impregnation method using a microwave-treated diatomite sample. The pentadecane/sepiolite composite prepared in the melting temperature range 7.98 degrees C to 8.53 degrees C and latent heat capacity range 41.05 to 46.02 J/g. The results of the thermal analysis indicate that fabricated diatomite-based or sepiolite-based PCM composites have good potential as thermal energy storage materials.