The relationship between the free-volume parameters and the permeability of organic compounds in polyacetals including different amounts Of -CH2CH2- comonomer units is analyzed by means of the positronium lifetime measurement. It is found that the content of the comonomer units affects not only the melting temperature and the degree of crystallinity but the free-volume size and fraction. The results suggest that the permeability of gasoline including methanol is controlled by size and fraction of the free-volume as well as the degree of crystallinity.