In their natural form, fats and vegetable oils melt at temperatures useful for thermal energy storage. Incremental improvement of their heat release characteristics could pave the way for commercial applications as phase change materials (PCM). These chemicals could provide a biomaterial alternative to a technology dominated by paraffin and salt products. Mixture heats of fusion and melting points were evaluated for single acid derivatives of stearic, palmitic, and oleic acids to develop a fundamental understanding of the solid liquid transition and to better understand how to convert natural compositions to useful PCM products. Many mixtures of monoacid derivatives of a single fatty acid formed single sharp solid-liquid phase transitions useful for PCM applications. These mixtures also qualitatively exhibited freezing point depression therein allowing the mixture composition to be used as a means to control the temperature of the phase transition. In mixtures of monoacid derivatives from different fatty acids, melting occurred over wider temperature ranges with eventual separation of the solid-liquid transition to at least two different events. This work demonstrates that it is possible to fully convert natural fatty acid mixtures into high performance phase change materials. (C) 2003 Elsevier Science Ltd. All rights reserved.