Composites made from an unsaturated polyester/styrene thermoset matrix and maleic anhydride esterified woodflour have been prepared and tested. Different degrees of esterification of the wood particles with maleic anhydride (MAN) were obtained by using different times of reaction, which led to materials with varied final properties. X-ray diffraction studies have been carried out on raw and chemically treated woodflours. The degree of crystallinity of modified woodflours is lower than that of the untreated one and decreases as the MAN content of the samples increases. The thermal degradation of the woodflours, matrix, and derived composites was estimated by thermogravimetry (TG) in nitrogen environment. Thermogravimetry was shown to be sensitive enough to detect differences between woodflour samples with different treatments and degrees of esterification and between composites with different woodflour contents.