Rapid thermal processing (RTP) was shown to be effective in reducing defects, especially grain boundary defects in c-oriented MFI membranes (Choi et al., Science 325 (2009) 590-593). In this study, the xylene and butane separation performances of RTP-treated c-oriented MFI membranes, which were now synthesized with a shorter secondary growth time (from 2 d to 1 d), were investigated. These membranes achieved a good performance for separating both p-/o-xylene (similar to 88 separation factor (SF)) and n-/i-butane (similar to 30 SF) isomers. In addition, fluorescence confocal optical microscopy was conducted to explore the defect structure by varying the contact time with the dye solution. This characterization revealed that the accessibility of dye molecules to grain boundary defects at the molecular level can be correlated with separation performance. Finally, the fluxes of xylene and butane isomers across RTP-treated c-oriented MFI membranes were deciphered by the Maxwell-Stefan formulation to obtain their diffusion coefficients. The diffusivities of the slow permeating component (i.e., o-xylene) were estimated to be lower than the literature values obtained by a zero length column method; this accounted for the high p-/o-xylene SF. We considered the combination of adsorbate-adsorbate and adsorbate-zeolite interactions to calculate the diffusivities of n-/i-butane isomers. (c) 2013 Elsevier B.V. All rights reserved.