The gas transport properties of polymethoxyaniline (PMANi) and polyethoxyaniline (PEANi), in which the methoxy and ethoxy groups mere introduced onto the benzene rings at the ortho position, were studied to determine the effect of substituents. Measurements were carried out at 40 psi from 25 degrees C to 55 degrees C. When compared with polyaniline, the gas permeability increases with the size a substituent because of the increases in the diffusivities while the selectivities remain the same or Increase for some gas pairs. Excellent correlations were found between the diffusion coefficients and the molecular diameters of gases and between the solubility coefficients of gases and their boiling points (or critical temperatures).