We have found that sharply defined unit-step morphological changes can be created in the middle of the cross-section of hollow fiber membranes wet-spun from highly concentrated immiscible poly(2,2 ' -m-phenylene)-5,5 ' -polybenzimidazole (PBI) and polysulfone (PSf) blend solutions. A halo is observed with yellowish color having wavelength in the range 580-595 nm. The abrupt change in transmittance light implies distinct morphological changes at the interface because the halo region possesses significantly different morphology and pore sizes from the inner and outer regions. The width of the halo ring is approximately 18-20% of the hollow fiber wall thickness. The mapping image and spectra obtained from field emission scanning electron microscopy coupled with energy dispersive X-ray analysis (FESEM-EDX) suggest that the distribution of the elements present in hollow fiber is homogeneous. X-ray photoelectron spectroscopic (XPS) results indicate that the halo formation is not caused by the phase separation of PBI and PSf, but by a physical phenomenon.