Using thermogravimetric analysis (TGA), the thermal behavior of sisal fibers and sisal/polyester composites, fabricated by resin transfer molding (RTM), has been followed. Chemical treatments have been found to increase the thermal stability, which has been attributed to the resultant physical and chemical changes. Scanning electron microscopy (SEM) and infrared (FT-IR) studies were also performed to study the structural changes and morphology in the sisal fiber during the treatment. The kinetic studies of thermal degradation of untreated and treated sisal fibers have been performed using Broido method. In the composites, as the fiber content increases, the thermal stability of the matrix decreases. The treated fiber reinforced composites have been found to be thermally more stable than the untreated derivatives. The increased thermal stability and reduced moisture behavior of treated composites have been correlated with fiber/matrix adhesion.