The degree of compatibility between poly (lactic acid) (PLA) and different bio-fillers was examined using thermal methods. The biofillers were fibers extracted from cuphea and lesquerella seeds. Bio-composites of PLA:Fiber were prepared at 85 : 15, 70 : 30, and 100 : 00 and blended by extrusion and then injection molding. Thermal properties of the extruded (EX) and the extruded-injection molded (EXIM) composites were examined using differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA). Composites analysis using DSC provided information essential for determining: (1) glass transition (7,); (2) crystallization and melting temperatures and Delta H; (3) percent crystallinity; and (4) Enthalpic Relaxation (ER). TGA was used to measure the mechanism of the thermal decomposition of the composites. The effect of fiber composition, processing conditions, and aging time on the thermal properties of the bio-composites was used to examine the relative compatibility of the fibers with PLA. The data showed that ER increased steadily as a function of aging. Because of the difference in protein content between cuphea and lesquerella, the two fibers influenced the T-g (temperature and Delta C-p) differently. The percent crystallinity of neat PLA was significantly reduced by EX or EXIM, while the enzymatic degradation showed that EX composites are more biodegradable than EXIM. The TGA profiles indicated a multi-step degradation especially in air. The largest value of q, which indicated that, intermolecular interaction was strongest between lesquerella and PLA extruded materials, compared to the other three blends, possibly due to the higher protein content. Results also indicated that intermolecular interaction was least strong (q = -33.9156) in the EXIM PLA-cuphea. Although the q values of both cuphea composites were significantly lower than those of the lesquerella samples, this greater effect of extrusion over injection molding on intermolecular interaction was also seen in the q value of the EX and EXIM samples. (C) 2008 Wiley Periodicals, Inc.(dagger) J Appl Polym Sci 111: 114-124,2009