 This paper examined the relationship between cellulose crystallinity, extractive content, and the physical properties of 10 different natural fibers. It was found that higher levels of extractives are associated with lower crystallinity and smaller crystallites, which leads to faster degradation and reduced thermal stability. Additionally, it was observed that the thermal decomposition of natural fibers is shifted to higher temperatures, as their crystalline structure increases. This article was authored by Mateus Paleto, A. Torrell, and Adémir J. Zadra.