The addition of particulate fillers (woodflour) and short fibers (sisal) into an unsaturated polyester matrix was performed and analyzed. The efficiency of the filler treatment was carefully investigated, in particular, esterification with two different anhydrides, maleic anhydride (MAN) and an alkenyl succinic anhydride (ASA). The reaction with MAN was performed under different experimental conditions in order to reach different degrees of esterification. The efficiency of the reactions was assessed by FTIR, titrimetric techniques and moisture absorption values. Esterification improves the wettability of the fillers by the resin, so that higher concentrations of filler could be incorporated in the composite. The reaction of the unsaturations of the attached anhydrides with the styrene co-monomer was confirmed by FTIR. This reaction was frequently invoked in the literature, but most times was not confirmed. Scanning electron microscopy study confirmed the improved adhesion of the fillers to the matrix. Surfaces of fractured esterified woodflour composites showed that the resin fills the hollow central regions of the woodflour cells (lumens) reducing capillarity effects during humidity or water sorption by the composites. Mechanical tests were performed on some of the composites to illustrate the effect of the covalent linkages created between the esterified lignocellulosics and the matrix. Tests in which the characteristics of the interphase played an important role were chosen in order to highlight differences resultant of the chemical co-reaction.