In this work, two aromatic carboxylic acids (benzoic acid and isophthalic acid) are proposed to modify the permeability of poly(amide-b-ethylene oxide) or Pebax for CO2 separation membranes. The effect of acid content on the gas separation performance of flat membranes was investigated using CO2, CH4 and N-2 at different pressures (4-10 bar) and temperatures (35-65 degrees C). The results show that both aromatic acids substantially improved the CO2 permeability (about 120% above neat Pebax) due to the strong interaction between Pebax and the carboxylic groups of the acids, while CH4 and N-2 permeability only slightly increase due to the restricted polymer chain mobility. This behavior led to enhanced CO2/N-2 and CO2/CH4 selectivity by about 55% and 80%, respectively. Fourier transform infrared spectroscopy (FTIR) confirmed the coordinative interaction between the amide group of Pebax with the carboxyl functional group of the acids. The membranes thermal and mechanical stability were also determined by thermogravimetric (TGA and DSC) and dynamic mechanical (DMA) analyses. Finally, benzoic acid was found to be of interest since it contributed to superior thermal and mechanical strength as well as enhanced CO2 separation properties.