In this article, for the first time in the literature effects of phenylene diisocyanate (PDI)-based compatibilizer on the physical and chemical properties of citric acid (CA) modified thermoplastic starch (TPS)/poly(lactic acid) (PLA) blends were investigated with respect to PDI and CA content and blend composition. The blends were prepared by melt compounding in a laboratory microcompounder. Fourier transformation infrared spectroscopy results showed that CA interacted with starch and PDI interacted by both starch and PLA through the hydroxyl groups. It was revealed from SEM micrographs that combinatorial usage of CA and PDI resulted in an improved, finer distribution of TPS in PLA matrix. This improvement affected the mechanical properties of blend, especially the toughness related properties such as impact strength and elongation at break. The thermal properties such as T-g and T-m revealed from differential scanning calorimeter analysis were in line with the morphological structure of the blends by suggesting the compatibilization phenomena in the presence of PDI and CA together. Thermogravimetric analysis showed that compatibilization of two phases improved the thermal stability of the blends. As a general conclusion, the combinatorial usage of PDI and CA can be utilized to obtain tougher PLA/TPS blends-based materials to overcome the brittleness problem. POLYM. ENG. SCI., 53:2183-2193, 2013. (c) 2013 Society of Plastics Engineers