A series of solid-liquid phase change materials, thiourea derivatives, were prepared via condensation of thiourea with the respective carboxyl chlorides (lauroyl chloride, myristoyl chloride, and palmitoyl chloride) and were then characterized by using FT-IR, (1)H-NMR, differential scanning calorimetry (DSC), and thermogravimetric (TG) analysis. The thiourea derivatives (1,3-didodecanoyl thiourea, 1,3-ditetradecanoyl thiourea, and 1,3-dihexadecanoyl thiourea) were structurally symmetric and had long alkyl groups to crystallize. Thermal analysis by DSC and spectroscopic investigation by FT-IR spectroscopy were performed on the samples before and after thermal cycling tests to determine thermal reliability. The maximum latent heats of melting and freezing of the thiourea derivatives were found to be 114.6 and -110.0 J/g for 1,3-didodecanoyl thiourea, 119.5 and -122.4 J/g for 1,3-ditetradecanoyl thiourea, and 148.8 and -142.7 J/g for 1,3-dihexadecanoyl thiourea after accelerated thermal cycling. A TG instrument was used to determine the starting point of degradation in the thiourea derivatives; it was found that the thiourea derivatives degraded at sufficiently higher temperatures than the expected utility temperatures.