We are testing the compatibilization of the chitosan/PLA blends by addition of diisocyanate and at studying the effect of several MDI concentrations (0.5 and 2.5% of the global blend mass, w/w). To evaluate the MDI efficiency as a compatibilizer of chitosan/PLA blends, we worked with the following methods: IRTF spectra with higher peak at 1558 cm(-1) is due to the -NH bonds that exist in urea and urethane, thermal properties shows that the temperature of the endothermic peaks of the chitosan/PLA blends with MDI is very close to the temperature of pure chitosane and SEM micrography shows that MDI addition decreases the PLA particles size in the chitosan mixture; they also comply with the compatibilization theory. After that the mechanical properties have been characterized: we can notice that the MDI compatibilized chitosan/PLA blends have a higher Young's modulus than the noncompatibilized blends. we are showed that the use of 0.5% MDI is not enough sufficient to obtain a compatibilization, because a part of the MDI can be consumed by water. The addition of MDI increases the performance of the mechanical properties of the blends. Therefore, with this compatibilization, we could obtain some chitosan/PLA blends that would be water-resistant and that would also keep their mechanical properties. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3083-3091, 2010