Although carbon surfaces are generally hydrophobic, the surface oxygen-containing groups act as water molecule sorption sites, reducing the performance of carbon molecular sieving (CMS) membranes over time. In this study, a hydrophobic poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) layer was coated on the surface of PPO- or polyetherimide (PEI)-derived CMS membranes to protect against aging. The permeation properties and anti-aging characteristics of the modified CMS membranes were explored using a single gas permeability test. Furthermore, thermal annealing below the PPO's glass transition temperature was applied to the hydrophobic layer to evaluate its anti-aging effect. The results indicate that the aging of CMS membranes was also determined by the carbon matrix structure and that a smaller pore size could prevent water adsorption and O-2 chemisorption. After the hydrophobic layer was post-treated with thermal annealing, the gas diffusion resistance was diminished, and the initial permeance was retained to approximately 60%, showing slight loss within the testing period. (C) 2015 Elsevier B.V. All rights reserved.