Time-resolved X-ray with Synchrotron radiation as light source is a powerful tool to study polymorphism or structural changes in food systems. First of all, in edible lipid systems it has provided precise information on the structural changes of the fat crystals at a time scale of 10 sec. Polymorphic forms present in a specific time may be characterized and quantified, and therefore, fruitful information on the kinetic and molecular aspects of crystallization and mixing processes of the various types of mixed-acid triacylglycerols may be obtained. This information cannot be obtained with the traditional thermal and structural techniques because of their complicated structural properties. Among X-ray applications in fat systems, it may be mentioned the study of new fats developed as alternatives to trans fat. Recently, the food and drug administration (FDA) announced its preliminary determination that partially hydrogenated oils, the primary dietary source of artificial trans fat in processed food, would not be "generally recognized as safe for use in food". This decision of the FDA makes it even more necessary to find alternatives to trans fats. Among the strategies developed there is a new variety of high stearic high oleic sunflower oil (HSHOSFO) coming from seeds obtained through conventional breeding techniques (i.e. non-genetically modified organisms) which is cultivated in Buenos Aires Province in Argentina. The main difference between fatty acid profiles of HSHOSFO and the sunflower oil traditionally planted in Argentina lies in the stearic content (18% vs. 4%) and oleic content (69 vs. 23%). Furthermore, HSHOSFO displays very low amounts of polyunsaturated fatty acids and about 4% of very long chain fatty acids in the form of arachidic (20:0) and behenic acids (22:0). Although HSHOSFO is a liquid, it may be fractionated obtaining solid fractions named stearins that proved to have good properties for different applications in food. This chapter highlights recent research on polymorphism of high stearic high oleic sunflower oil stearins developed as trans fat alternative and the effect of processing conditions and additives on polymorphic behavior with the aim of increasing its functionality in different products.