We have observed time-dependent drifts in permeability and selectivity for two types of composite hollow fibers used for air separation. One was PVP [poly(4-vinyl pyridine)]/ 6FDA-durene/polyacrylonitrile (PAN) composite hollow fiber, and the other was 6FDA-3,5-diaminobenzonitrile/6FDA-durene/PAN composite hollow fiber. Their permeabilities dropped 50 to 70% after 3 to 5 months, while selectivities for O-2/N-2 deteriorated slightly with time. A systematic study was carried out to investigate the causes of this creep behavior. Various composite fibers, such as polyimide/Celgard and polyimide siloxane/PAN, were fabricated to simulate the aging process. We conclude that the aging phenomenon observed for these two 6FDA-durene/PAN composite fibers was not due to the structure change of the PAN substrate, but mainly to the densification effect of the 6FDA-durene gutter layer on composite fibers.