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This paper presents a study on the characterization of density as a function of temperature for phase change materials (PCMs). More specifically, in this study we analyze organic alkane PCMs, often called paraffins. PCMs are materials that have the ability to absorb a substantial amount of heat during phase transition from solid to liquid, and therefore prove to be useful in thermal energy storage. The density of paraffin wax PCMs is largely dependent on temperature, and during the phase change process, the density decreases dramatically as the PCM transitions from solid to liquid. Consequently, the PCM experiences dramatic volumetric expansion during this transition. Besides the thermal energy storage uses of PCMs, this volumetric expansion that they exhibit is also used in thermal actuator applications, often referred to as wax motors. While density of PCMs does affect their thermal and mechanical performance, the property is not well-characterized within the literature. In this paper, we examine ten paraffin wax PCMs with varying meltingtemperatures and characterize their densities as a function of temperature. This characterization was done usinga piston and cylinder dilatometer test setup within a temperature-controlled thermal chamber that we designedand validated to the well-characterized density properties of water. The density and temperature relationships werefurther analyzed using piecewise linear regression analysis to develop mathematical models of density as it relates totemperature, which will be useful to those wishing to analyze designs in which PCMs are used, such as in PCM-filled heat sinks.