Demands for engineering applications of high performance polymers, such as the polyaryletherketone families, have grown significantly in recent years. Fundamental knowledge on the mechanical behavior of such polymers, especially under harsh environments, is still lacking. In this study, the mechanical behavior and interfacial phenomena of a blend of polybenziniidazole (PBI) particles having sizes of about 50 mu m and polyetheretherketone (PEEK) matrix at 50:50 weight ratio was characterized. PBI and PEEK are known to be incompatible with each other in the dry state. The possible influence of hygrothermal exposure on interfacial bonding between PEEK and PBI has been systematically studied. Under dry conditions, the PEEK matrix dominates the dynamic mechanical behavior, indicating no noticeable viscoelastic coupling between the two phases. After being exposed to hygrothermal conditions, the T-g of the PEEK phase in the blend shifts to a lower temperature by as much as 26 degrees C. The extent of this shift is much greater than what is observed for neat PEEK alone under the same condition. Nanomechanical property mapping results indicate that the interfacial region is broadened after 288 degrees C hot water immersion treatment. The above finding strongly suggests the hygrothermal conditioning promotes apparent compatibility between PEEK and PBI. Likewise, the fracture behavior of the blend provides direct evidence that the interfacial adhesion is greater than the cohesive strength of either phase after water immersion. In the meantime, it is found that both fracture toughness and tensile strength of the blend are improved by water absorption at 60 degrees C, at the expense of a minor reduction in Young's modulus. However, if the blend is exposed to hot water at 288 degrees C, all of the mechanical properties will deteriorate significantly. The possible causes of the mechanical property deterioration in the PEEK/PBI blend after hygrothermal treatment are discussed.