Organic-inorganic hybrid (mixed matrix) asymmetric hollow fiber membranes were spun via a dry jet-wet quench procedure using surface modified inorganic small pore size zeolite incorporated in an Ultem((R)) 1000 polyetherimide matrix. The zeolites were modified via two separate techniques and termed as (1) Ultem((R)) "sized" and (2) Grignard treated. The first technique failed to achieve successful mixed matrix performance, but the second approach gave very attractive results. The Ultem((R)) ''sized" zeolites were prepared by treating the zeolites with a silane coupling agent to allow Ultem((R)) polymer chains to be grafted to the surface. Poor adhesion was observed between the bulk polymer and most of the zeolite particles in the final membrane using Ultem((R)) "sized" particles. The post-treated fibers did not display enhanced selectivity over neat polymer with pure gas nitrogen, oxygen, helium, methane and carbon dioxide testing or mixed gas carbon dioxide and methane gas pair. The absence of the mixed matrix effect is hypothesized to be due to the nucleation of hydrophilic solvent and non-solvent around the Ultem((R)) ''sized" zeolite particles during phase separation in the quench bath forming a so called sieve-in-a-cage defect. Although such defects have been reported in dense mixed matrix films, they have not yet been investigated in hollow fibers format which are formed via a phase separation process and thus remain prone to the effects of the non-solvent quenching media. To test the nucleation hypothesis, fibers incorporating 10.3 vol% (with respect to polymer) of zeolites modified with a Grignard reagent were spun. These fibers, after post-treatment, showed significant selectivity enhancement of 10% for O-2/N-2, 29% for He/N-2, 17% for CO2/CH4 pure gases and 25% for mixed gas CO2/CH4 pairs over neat polymer results. The selectivity properties in these fibers are similar to or exceed the Maxwell model predictions for these hybrid materials. (c) 2006 Elsevier B.V. All rights reserved.