The effect of surface fluorination on the ps transport properties of composite membranes, comprising an inert porous ceramic support and a selective layer consisting of poly (phenylene oxide), was examined. A small reactor volume permitted the treatment time and the fluorine feed concentration to be investigated independently. The gas transport properties of the treated membranes were evaluated for six gases (N2, O2, CH4, H-2, He and CO2), in terms of permeance (P/l or pressure-normalized flux) and the ideal selectivity for eight pairs of these gases. It was generally found that fluorination at different fluorine feed concentrations and reaction times reduced the permeance of all of the gases. The permeance of the lighter gases (He and H-2) was reduced by a smaller factor than that of the heavier gases (N2 and CH4). Fluorination increased the selectivity of He and H-2 relative to N2 or CH4 by a small factor, but reduced the selectivity of O2 and CO2 relative to N2 or CH4. When the membranes were coated with a layer of poly(dimethylsiloxane) (PDMS) subsequent to fluorination, the permeance decreased, considerably more for N2 and CH4 than for the other gases. Surface coating also substantially increased the selectivities of all the gas pairs. The largest gains in selectivity after fluorination and coating were found at the higher concentration (0.1% F2) and intermediate treatment times of 3 to 5 min. Based on these results, surface coating with PDMS is recommended as a post-treatment step in the fluorination process.