We have studied the thermal and mechanical properties, miscibility and morphology, and separation performance of polybenzimidazole (PBI)/polyetherimide (PEI) composite hollow fiber membranes. These composite hollow fibers were wet-spun from dopes containing 25.6 wt% solids in N, N-dimethylacetamide (DMAc) with different PBI/PEI ratios. Water was used as the external coagulant, while either H2O or DMAc/H2O mixture was employed as the bore fluid. Thermal mechanical analyzer (TMA) data indicate that these composite hollow fibers are miscible blend membranes. The molecular interaction between PBI and PEI are so strong that their miscibility appears to be independent of bore fluid chemistry, bore fluid flow rate, methanol and hot ethylene glycol treatments. The Tg values of wet-spun PBI/PEI blend hollow fiber follow the theoretical prediction of the Fox equation. Both SEM photographs and gas permeation data indicate that an increase in PBI percentage in the spinning solutions resulted in a hollow fiber with a tighter morphology, a lesser layer of finger-like voids and a significantly lower gas permeance. The tensile strength of wet-spun PBI/PEI blend hollow fiber membranes seems to be independent of PBI concentration, while their elongation at break decreases with an increase in PBI concentration.