Certain natural fibers have been attracting attention in order to mitigate the consumption of fossil fuels toward partly or totally replacing glass fibers in polymeric composites. The main organic sources for these fibers are the following plants: flax, hemp, sisal, curauá, coir, jute, bamboo, and banana. Hybrid composites with two different natural fibers can also be manufactured in order to potentialize certain advantages of each fiber. Furthermore, certain thermal properties can be enhanced using chemical treatments (such as alkalinization and benzoylation) or chemical coupling agents (like maleic anhydride and silanes) onto the fibers in order to improve their compatibility with the polymeric matrix. This chapter discusses the main findings reported in the literature on the thermal properties of hybrid composites produced with chemically treated natural fibers, coupling agents, or both. Improvements in thermal stability are commonly reported for both thermoset and thermoplastic composites. Biobased coupling agents should be developed to replace maleic anhydride and silanes.