We have revealed, for the first time, the evolution of nano-particle distribution and the mechanisms of nano-particle movement during the formation of single-layer and dual- membranes. Both single-layer and dual-layer mixed matrix hollow fibers have been fabricated using nano-size zeolite beta. It is found that three factors play important roles in determining the nano-particle distribution in the resultant fibers. They are (1) shear within the spinneret, (2) die swell when exiting from the annulus spinneret and (3) elongation drawing in the air gap region. The shear stress induced within the spinneret determines the initial particle distribution. As a result, the particle distributions in both wet-spun single-layer and dual-layer hollow fibers follow the parabolic shape of dope's axial velocity profile in the spinneret. Die swell flattens and levels off the particle distribution, while elongation stretch redistributes the particles to the outer surface with the aid of (1) moving outer surface inward and (2) the outflow of solvents during the phase inversion process. The particle redistribution phenomenon is significantly magnified and enhanced during the co-extrusion of dual-layer hollow fiber membranes with an outer layer made of mixed matrix materials. The proposed causes and mechanisms for particle distribution have been verified by the EDX line scanning spectra of silicon and SEM pictures. Preliminary gas separation data indicate that the outer mixed matrix layer of the as-spun hollow fiber membrane has a defective skin for Knudsen diffusion. Further post-treatment is needed to eliminate defects for gas separation. (c) 2004 Elsevier B.V. All rights reserved.