It has been demonstrated that a novel p-xylenediamine/methanol soaking method could efficiently remove the polymer-zeolite interface defects of the mixed-matrix structure. In this work, the mixed-matrix structure is in the form of an ultrathin (1.5-3 mu m) polysulfone/ zeolite beta mixed-matrix layer that is supported by a neat Matrimid layer in dual-layer composite hollow fibers. The particle's loading in this thin layer has reached 30 wt %. The ideal selectivities of the mixed-matrix hollow fibers (30 wt % of zeolite) for O-2/N-2 and CO2/CH4 separation were roughly 30 and 50% superior to that of the neat PSF/Matrimid hollow fibers, respectively. Investigation of the morphology of the mixed-matrix selective layer and its relation with gas separation performance indicated that without p-xylenediamine/methanol solution treatment, the outer layer showed various polymer-zeolite interface structures in different fibers with the same heat treatment procedures; this situation might lead to the different selectivities after coating. However, by applying p-xylenediamine/methanol processing on the fibers before thermal treatment, the fibers obtained a more uniform structure and improved attachment between polymer matrix and zeolite surface. Hydrogen bonding was proposed as the possible mechanism for the tighter attachment between the two phases. The improvement of separation efficiency was presumably related to the polymer chain rigidification, partial pore blockage, and/or favorable interaction between the gas penetrants and zeolite framework. (c) 2006 American Institute of Chemical Engineers.