Polymorphism is a specific feature of the triacylglycerols. The crystallization and melting properties are essential for the characteristics and industrial applications of fats. Positron annihilation lifetime spectroscopy（PALS） allows for the characterization of microstructural changes in triacylglycerol crystals at the atomic scale. When positrons enter molecular materials，they form positronium（electron-positron bound states） with surrounding electrons，whose lifetime reflects the size of molecular-voids（free volumes）. In this paper，PALS was used to study the polymorphism and phase transition processes of five saturated monoacid triacylglycerides for the first time，including trilaurin，trimyristin，tripalmitin，tristearin and arachidin. The patterns of crystal phase transitions obtained from the PALS are in good agreement with those from other characterization methods. The positrons experiments revealed the formation of voids during the solid phase contraction of longer-chain triglycerides into more stable crystal forms，as well as the transformation from the

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